None. I like aging and growing. I'm not the same person I was two years ago, and two years from now, I'll be different again.
If I had to pick an age to physically stay at, yet still get more mature? For women - mid-20's to mid-30's. That is when women are in their prime, before age starts giving problems like osteoperosis and loss of muscle mass. Also, society unfortunately tends to favor youth in women. I don't believe in this... I think women get more beautiful as they age... but there are probably more opportunities available to a younger woman.
If I were a man... probably mid-30's to mid-40's. Men are so good looking in those years. They are still in their prime, and usually have an air of distinction and authority without being too old to worry about the advisability of starting another career.