For appearance sake I would love to stay age 25 or 30. However I would want to have the wisdom that I have today.
Ultimately though life always comes down to the relationships we form over the years. I imagine it would become (extremely depressing) going to funerals of those who aren't able to preserve youthfully forever.
What sounded like a blessing could eventually feel like a curse as you watch your spouse die, friends die, children die, grandchildren die, your second and third spouse and so on.
Eventually you may try to avoid getting "close" to people. Loneliness leads to depression and depression can lead to thoughts of suicide. However if you knew it wasn't even possible to kill yourself then life may feel like a living hell! Death is a "graduation" of sorts or a "transition" for moving on. Feeling (left behind) would be torture.