Let’s do the time warp again · 9:34pm Oct 16th, 2019
Big Jim Miller solicited questions on Twitter once the finale aired. Interestingly, a lot of his answers revolved around the fact they deliberately left a lot of things vague or unresolved explicitly to give the fandom fodder for discussion, interpretation, and following out—specifically including fan fiction, if I recall correctly. One of those items was the precise amount of time elapsed between the finale and the epilogue. So here’s my take, but be aware I haven’t bothered with spoiler-blackout on the text because that would be tedious.
Twilight has grown into a Celestia-like stature. What this means for her lifespan is not clear, though I still prefer the idea she will live a long and healthy, but not immortal, life (per occasional hints from, among others, Meghan McCarthy). The rest of the Mane Six show noticeable signs of aging, which some have interpreted as meaning many decades have passed. I think, however, based on the apparent ages of the CMC (and the Cake twins, as ZanyZapper points out) and—especially!—next-generation foals, only ten or fifteen years can have gone by, which would put Twilight and her friends in their thirties. The seeming discrepancy, I believe, rests in the peculiarities of the simple design and animation styles adopted for the series.
Other than a larger, beefier Spike, Twilight’s friends use the same body models they’ve been animated with throughout the series, as far as I can tell. Said models, the same basic design used for most adult mares, were an early exigency dictated by the degree of overwork the art and animation staff already faced; there was no additional time or budget to create unique individual models for the Mane Six, as Ms. Faust originally desired, as well as those already required for other recurring characters.
Those generic models appear, deliberately I’m sure, more or less ageless, able to support any stage of life, from adolescence to middle age, required for a particular character. (Consider, for instance, that Twilight Sparkle, Starlight Glimmer, and Stellar Flare all use pretty much the same body design, despite what has to be an age range of twenty to thirty years!) For this reason, I suspect the designers and animators resorted to a bit of exaggeration, most notably in altering the styles and colorations of manes and tails, to make the passage of time glaringly obvious. On the other hand, their faces are altered only slightly, with nothing more than the ubiquitous “eye bag” tic marks showing any form of age wrinkling.
If one needs a rationale to reconcile it all, I suggest a life of adventure and helping to govern the nation can be a bit hard on the constitution. 
I was thinking about the same, maybe 15 to 20. Mainly due to the Cake twins looking late teens / near adults to me. Tho that also puts Pinkie and Sugar Belle up there a bit when they had their foals. But a lot of human women give birth in mid to late 30s so might not be much of a stretch. Ages and time in MLP have always been a bit nebulous!
5141066
When it comes to Pinkie and Sugar Belle's kids there's some common speculation that the two we saw were the ones young enough that they still needed parental supervision. They could have older kids off doing their own thing.
5141198
Fair enough, though that doesn’t address the CMC being shown as young adults.