Ianto Jones: Growing Old You will never age for me, nor fade, nor die.
There's supposed to be a second part to that quote. Nor you for me. But there isn't. Not for him. Not for Jack. Jack will be able to give that to him, but he won't be able to do the same. Assuming he lives long enough for that to become a problem, which he doesn't really expect to. He hasn't really pictured himself growing old, and especially not since he joined Torchwood. It's just too much a high-risk job.
And what will happen if he does? Jack will watch him age and wither away, become an old man with grey hair and wrinkles. But Jack won't age, won't change hardly at all. Maybe a year or two, over the whole span of Ianto's life. And while no one really looks forward to aging, some comfort can be gained from having someone to grow old with. Supposedly. That's what people say. It's just that the person he wants to spend the rest of his life with, well, has much more life than he does. It doesn't seem fair, but then, since when has life been fair?
It's fear, that's what it is. Fear of the unknown, and of facing it by himself. He doesn't really know what's coming; everyone he's known has either died young or aged prematurely. And as much as he trusts the fact that Jack will love him always, regardless of how old he is or what he looks like, how can he ask that of Jack? How can he ask Jack to stay with him like that?
His voice shakes a bit when he finally speaks. "I don't want to get old. No one does. But it beats the alternative." He pauses and almost sits down again before adding, "I hope."