Hmm, neither a hundred-year-old nor a ten-year-old are generally particularly strong physically, so if someone wanted to attack me I might have problems. However, if you only look that age but are actually not physically as weak as you would be at that age, I can't help but keep thinking of Babette from Skyrim and want to look ten years old (okay, so Babette is a vampire which is why she looks like a small child - because that's how old she was when she got bitten, but still...). I suppose it depends whether the physical/mental infirmities of old age actually show up, or whether I just look all wrinkly and stuff. After all, if I'm only gonna look that age, maybe I can still be all there underneath.
But then, I wouldn't want to be immortal in the first place. After all, to live forever is to ultimately be alone forever, unless you can find someone else who's also immortal, and then you'd probably get fed up of each other after a few decades/centuries/millennia...