Simple vocabulary tests on children as young as two can identify those who will find it difficult to learn language and lag behind their peers in becoming fluent.

The doctors who developed the tests said they provided a crucial means of spotting children who could benefit from early language therapy to help them reach their full potential.

Though most children who are late talkers catch up by the age of five, a significant number have specific language impairments that can affect their ability to read and learn at school and in later life.

At the annual meeting of the American Association for the Advancement of Science in Vancouver, the researchers described how they had asked parents to complete a checklist of words their children spoke at the age of two and followed their progress for 15 years. In the study, 26 children were regarded as late talkers, while 23 had an average vocabulary for their age.

The checklist included 310 words, and parents were asked to tick off any they had heard their child say, even if the word was not perfectly pronounced or was used out of context or with another meaning.

Two-year-olds typically uttered between 70 and 225 different words, but late talkers only spoke around 50 words or fewer. Some of the earliest words all children picked up included "cat", "dog", "car", "shoe" and "no".

The researchers, led by Dr Leslie Rescorla, a psychologist at Bryn Mawr College in Philadelphia, asked the late talkers to take a series of language tests throughout their childhood years and into young adulthood. They found that while most caught up enough to have average language skills, they lagged behind a control group of peers at every stage of their development.

"The children recovered from their language delay at a varying rate. By four years old, more of them were in the average range, and by the time they were at school, most were performing in the average range. The not-so-good news was that after the age of 17, the children performed significantly less well on all language measures relative to a comparison group from same background," Rescorla said. The children were tested on vocabulary, grammar, verbal memory and reading ability.

Prof Nan Bernstein Ratner, a hearing and speech sciences expert at the University of Maryland, said that vocabulary tests on two-year-olds could identify children who were failing to pick up language because of poor hearing, autism spectrum disorders or mental impairment.

"Upwards of a fifth of children can be viewed as late talkers, but around 80% of those are just later bloomers and catch up," she said. "Some of these children will recover, but the problem is we don't know which ones. If you don't have a crystal ball it's much better to catch them at age two and start tracking them rather than waiting to see what happens.

"If you have problems with language and reading, you have children who will not succeed in society. The checklist is simple and it doesn't require machinery. All you need is a pencil and a willing parent," she added.

A growing body of research shows that children develop language more swiftly if they are engaged in real conversations rather than left to watch children's television programmes. The effect is thought to be related to the amount of direct interaction with others, rather than being a negative influence of television programmes per se.

"Children really need to have others talk to them in order for them to acquire language. It's not that they don't learn anything from television, but that they need a language partner," Rescorla said.