I take it you mean something like
a [measurable but indeterminate] period in the human lifetime after acquiring the wisdom of experience and before losing the flexibility of youth.
If this is correct, then I don't think horizon in its common sense works. As you note in a couple of comments, a horizon is a dividing line or border—one crosses it, rather than being in the midst of it. It's also typically a barrier or limit of a sort, since one cannot see beyond the astronomical horizon.
However, if you are talking to geologists or archaeologists, it might make sense. According to the OED Online, a horizon can be
a. Geol. A . . . stratum or set of strata characterized by a particular fossil or
group of fossils.
c. Archaeol. A level . . . which is taken as representing a particular culture or cultural
("horizon, n." Def. 5. OED Online. Oxford University Press, June 2016. Web. 28 August 2016.)
Basing your metaphor on one of these senses of the word, I don't think it's too big a stretch to refer to an "era" of your life as a horizon.