I suppose at 25. At 25 you get to the point where pretty much everything opens up to you, and it's not too old either. At 25 you are still considered pretty young and it's out of the 4-year college, so definitely that age for those reasons.
I would like to stop aging when I am 24, that time would be when I am in my prime, I would have all the skills necessary for life, and I would look good.