Two independent – almost in fact inverse – variables go into making a man’s best years: vigor and wisdom.
You have plenty of vigor at seventeen. Less so at near-fifty. Though in good shape, I can’t run a sub-5:00 mile like I used to. (I can still beat teenage boys up a steep hill sprint and do 50 perfect-form pushups with minimum effort.)
But you have very little wisdom in your youth. When it’s said that late forties are a man’s best years, they’re talking about the optimal intersection of vigor and wisdom. Slide that point back to thirty or forward to sixty as you wish.
It’s similar for women, except that their two variables are beauty and social affirmation. Beauty in their youth, social affirmation as they age if they do things right: children they are proud of, a stand-up man for a husband, some measure of material and spiritual comfort that they helped build. Then grandchildren.
(The crazed pussyhat “Refugees Welcome” spinsters are what happens when a woman’s beauty, such that it was in some cases, fades and all that they have in their middle age is an ugly face reflecting an ugly soul, barrenness, and other relics of their horrible decisions.)