The Older One is clearly the Real Human Sunset, which is possibly due to having some unusual side-effect happening to the Aging Process for beings from Equestria, like being stuck in the Human World, while stuck in a Human Form, causes them to Stop Aging, leaving them stuck looking exactly the same as they did when they first arrived, I mean the Sirens/Dazzlings were banished to the Human World over 1000 Years Ago, and yet they still look like mere teenagers, so both Sunsets could very well be somewhere in their Mid-20's/close to 30 Years Old, and yet Pony Sunset is stuck looking like she's barely even a day over 18, think of it with Attack on Titan Logic, the character Ymir is actually almost 80 Years Old, yet because she once spent so long as a Titan she looks as if she's not even 20 yet.