Introduction

One of the critical challenges in aging and longevity research and healthcare in general is the development of widely-available and reliable biomarkers of aging. Individuals can be of the same chronological age and have different biological ages. Biological age reflects functional capability and physiological status, whereas perceived age reflects the age other people guess a person to be [1]. Chronological age predictors can be used to identify the divergence between estimated biological or perceived age and true chronological age among people with accelerated or delayed aging [1]. It is important to develop non-invasive photographic biomarkers, since they are capable of providing valuable insights about the condition of the human body. Highly-accurate predictors of chronological age can also be used to evaluate the various lifestyle, medical, and cosmetic interventions.

Humans can predict the age of other humans with reasonable accuracy. However, people’s error rates vary across ethnic groups and from person to person. The many diseases as well as the general health status of the person are often apparent to the trained professionals, family members and untrained individuals. Humans without prior medical training can detect a variety of acute diseases using just facial images [2].

In order to investigate the biological relevance of photographic biomarkers of biological age, the accurate chronological age predictors must be developed and studied. In this study, the photographic images of human skin were used to predict age. Wrinkles and changes in skin pigmentation indicate aging making skin condition a reasonably good predictor of chronological age (here and below the age refers to the chronological age). Our main finding was that the photographic images of the skin around the eye can serve as a very accurate, non-invasive biomarker of the age. We also found that this photographic aging clock was able to estimate age with higher precision than the methylation aging clock commonly referred to as the Horvath’s clock [3]. Horvath’s clock is a DNA-based epigenetic measure which is considered to be a state-of-the-art aging biomarker. While age predictors such as the methylation aging clock are accurate in predicting the chronological age and multiple studies suggest the biological relevance of these clocks, there are some questions regarding the biological utility of and the relationship between these predictors [4].

There are numerous papers on age prediction with biomarkers; these papers cover a broad range of disciplines including biology, bioinformatics, machine learning and computer vision. The use of the facial images for age estimation is widespread. This approach is supported by a large number of images of faces and datasets of faces available on the internet, such as FG-NET [5], MORPH (the largest public face aging dataset) [6], Adience [7], and IMDB-Wiki [8]. The predictors trained on imaging data are commonly used for related tasks, such as: gender estimation, landmark estimation, and 3D model reconstruction. Most computation methods invented in the last decade rely on statistical models and manually-designed features [9–12], which are generally useful only for specific tasks [13]. However, with advances in computer vision, age estimation can even be done on unconstrained (in-the-wild real-life) images. Unconstrained images are images that may contain artifacts such as blur, occlusion, or various degrees of deformation. Deformation can occur from either strong facial expressions or head movement. Qawaqneh et al. [14] employed the VGG-Face network for age prediction. Their solution achieves high accuracy of age prediction on Adience in-the-wild dataset (59.9 exact accuracy and 90.57 1-off accuracy for 8 age groups). Zhang et al. [15] proposed a solution for simultaneous age and gender prediction with Residual Networks of Residual Networks (RoR) pre-trained on the ImageNet [16] and the IMDB-WIKI [8] datasets. Their solution achieves very high quality on the Adience dataset (66.7 exact accuracy and 97.38 1-off accuracy) with a 34-layer network. Rothe et al. [8] studied the tasks of chronological (real) and apparent (perceived by other people) age prediction. Their pipeline started with a face detector which determined the position of a face on an input image, the position was then normalized (without using facial landmarks). Next, classification occured via the convolution neural network (CNN) pre-trained on ImageNet dataset and fine-tuned on the IMDB-Wiki dataset. The authors reported the MAE of 3.318 years for apparent age prediction on IMDB-Wiki dataset and MAE 2.68 and 3.09 years for chronological age prediction on MORPH2, and FG-NET datasets, respectively. Another application of using face images as a biomarker of aging involves imposing age changes on the image using Deep Feature Interpolation [17] and Generative Adversarial Networks (GANs) [18]. It is possible to produce an older image of a person from a recent photograph with high quality results [17,18]. One of the popular approaches to age estimation is based on the analysis of various biological data types. For instance, Zhavoronkov et al. [19] predicted age by training the ensembles of deep neural networks (DNN) on the basic blood biochemistry data. These models were trained on over 60,000 blood biochemistry tests samples. The authors reported the values R2 = 0.82 and MAE = 5.55 years for chronological age prediction. The ensemble of DNNs also identified the five most important markers for predicting human chronological age: albumin, glucose and urea concentration, alkaline phosphatase activity, and erythrocytes number. Further studies of this approach helped establish its biological relevance by testing the modified predictor on the large population data sets including the data from the National Health and Nutrition Examination Survey (NHANES) and demonstrate that the DNN-based age-predictors can be population-specific by comparing the accuracy of the predictor in the Canadian, Korean and Eastern European Populations [20]. Age can also be predicted by DNA methylation rate, as was discussed in this seminal paper [21]. In this paper, using of 8,000 samples from 82 Illumina DNA methylation array datasets, encompassing 51 healthy tissues and cell types, allowed age to be predicted with R2 = 0.96 (between methylation-base predicted age and true chronological age) and MAE = 2.7 years. Despite high precision, the main disadvantage of DNA methylation-based methods is their invasive nature [5,21].

Our work is devoted to usage of deep learning approach for accurate chronological age prediction and investigation of features contributing to age prediction. This method only requires a single high-resolution photo of the corner eye area. The eye corner area of the human face is believed to be the most prone to aging [22]. Therefore, we believe it holds important clues for the creation of photographic aging biomarkers.