They've talked about it before, Jack and Ianto, and Jack tried to reassure Ianto that it really was okay to age and he hoped Ianto lived long enough to do that. He's not sure how much it really sunk in, though; it's a deep fear that isn't easily persuaded away. There's no "growing old together" there.

Maybe Ianto could do that with Alex, if they stayed together that long, maybe not. Their lives involve a lot of risk and danger, after all, so even with a commitment to stick together like that, it might not happen. Alex has probably made a lot of enemies over the years and Ianto is Torchwood. And Jack has to vaguely wonder what would happen if Alex realized that Jack wasn't aging like everyone else. He hopes the rest of Ianto's life is awhile yet, longer than might be expected for someone in Torchwood, though that's a lot to hope for.

He offers Ianto an encouraging smile when Ianto finishes speaking. He knows it had to have been hard for Ianto to say anything about it, hard enough for Ianto to talk about it with him.