As a human Connie has a finite lifespan. She will die eventually. Steven could turn her pink like Lars but Connie would still have to die first. There is the possibility that Stevonnie could have a long lifespan but Steven and Connie would have to become a permafusion. But the show hasn't addressed this issue because Steven is completely unique and the full extent of his powers is unknown.
I mean if theres the whole thing of Steven ages only as old as he feels surley seeing people start to grow up around him would help? Like in his birthday episode he managed to get a bit of facial hair at the end of the episode becasuse he learnt the moral. He's not stupid surley he'd match Connies age if he saw her graying up, preferabley sooner then later
I'm sure Steven can age appropriately with Connie, Greg or anyone he chooses. It's the fact that Steven is going to watch all of his human friends die while he lives on. That's going to be an immense psychological burden for him to bear.
"age manipulation" isn't actually one of steven's powers, in so many birthdays he was shapeshifting his body in accordance to how he felt, this is confirmed by the usage of an identical pink glow effect
he shapeshifted into an old man, so his organs began to fail accordingly, so he couldn't keep himself immortal by doing this because eventually the strength to shapeshift young would wear out
i guess its more of a subskill of his shapeshifting that can cause aging and senescence which wouldnt be the strangest thing his shapeshifting can do. after all he made his fingers into creatures with their own wills separate from his own(i guess from shapeshifting multiple brains into gis fingers)
yeah the thing is steven's body is biological so his shapeshifting has biological consequences (if he creates the structure of a failing body, the body fails; if he creates the structure of a smaller living organism, it has its own sentience; etc)
i think its very possible for him to be biologically immortal thanks to the power from his gem and healing powers it seems like an easy feat(we do have a few examples of basically immortal life on earth so a gem powered lifeform could do way better)