For me, probably around 28 years old. You're young, healthy and much wiser than when you were in your early 20s. You have ambition to create something in the world but aren't totally naive like a teenager.
This is an interesting question. But it contains a false premise. Physically, I'd obviously want to remain a young forever. But if this weren't a Groundhog Day scenario, it would be impossible to remain the same age mentally or socially forever.
If the question were reformulated as What Age Has Been Or You Believe To Be The Best In Your Life, I would answer 25 to 30. Childhood and teenage sucked. As a young adult between 19 and 25 I was too naiive to make the best of it. Between 25 and 30, I was less naiive but still felt I had a lot of potential. At that age, a person is not yet fully committed to a career choice or a serious relationship but begins to enter into adult life. That stage in life is so crucial that many would benefit from better mentoring.