Age is Just a Number

If this study is accurate then I'll go with 25.

From Science Daily...

"How old you are matters, but beyond that it's your interpretation that has far-reaching implications for the process of aging," said Markus H. Schafer, a doctoral student in sociology and gerontology who led the study. "So, if you feel old beyond your own chronological years you are probably going to experience a lot of the downsides that we associate with aging.

"But if you are older and maintain a sense of being younger, then that gives you an edge in maintaining a lot of the abilities you prize."