I would be 25 forever, so long as I could still die if I wanted to somehow. I don't want to be left floating in the nothingness of space after a few trillion years when the earth is destroyed by our sun exploding.
I would like to live for as long as I want, which I know will not be forever but long enough in a good fresh physical state, however I could age at the speed I wanted to too. So perhaps, maybe 500 years or more.
In a physical body with phsycial, I don't know if I would live here on this earth for a long time. Maybe just until Im not entertain with my self or get the option of death. If I was the only one, its a lonely life.
Especially taking account for people who live immortal on the earth in many movies, and animations. They would be impervious to dying, but after awhile, for certain people, it can lead to a lonely life. For one, which part is immortal. If the brain is pretty still physical, our brain still ages.
In fact, cracked list all the reason why immortality would be worse then death. [link]
If immortality came with people who were immortal, then i probably would. Though like Kill said, I could argue I have immortality in my hands.
Given that I don't believe that a glimmering happy paradise in the clouds awaits me after I bite the dust, heck yeah I'd be immortal. As for which age, I'd go for 21. You're young, but you're not too young.