If you could make one (1) artist immortal and ageless...?
Yes, I saw the episode of Highlander when that girl lost her artistic ability when she became immortal. So let's specifically avoid that in the spell. :P
One artist, singer, musician, composer, etc. Who will be eternally young. Not invulnerable, but still unable to die through old age.
He or she will constantly improve, albeit at a slower rate. Give him/her enough time, and s/he will become the greatest ever.
This also means that s/he will outlive you. You will never be able to see him/her reach his/her full potential.
One catch: if he or she ever gets tired of it all, the spell will be undone and s/he will return to his/her age as of today, and will continue aging as normal.
So. Who will be the artist you want to live forever?
Only artists born BEFORE 1990, please. If you're an artist, sorry but you can't do this to yourself.