Castiel CHOOSING to let his vessel age alongside Sam and Dean. He first excuses this by convincing himself he's only doing it to not stand out and it's saving his grace. That it's wasteful to constantly keep rejuvenating cells when he could be using that power elsewhere. It simply makes the most sense, especially when he falls from Heaven to help aid Dean Winchester. The years go by and the vessel is no longer just a vessel, it's his body and he likes experiencing the human process of aging. The wrinkles he's getting at the corner of his eyes, the laugh lines on his cheeks from smiling at Dean's antics, the grey hairs starting to dust his hair. He loves experiencing this part of humanity. Sam and Dean don't notice how odd it is Castiel is aging alongside them. They're human, so they don't think anything of someone aging until it's brought to their attention by someone Castiel is an angel-- vessels don't age. Or at least, they shouldn't. Dean thinks something has to be wrong with Cas' grace, that Cas has been hiding he's hurt or something fatal but Cas tells him there's nothing to worry about. He chose it. He could at the snap of his fingers restore the body back to its original state when he first possessed Jimmy Novak but he doesn't want to. Dean asks why doesn't he. I mean, aging isn't exactly glamourous and it's something a lot of humans would want to skip out on. Cas answers by saying, "I wanted to because... because it makes me feel more like I belong", but Dean notices his stutter. Cas' answer is satisfactory, but Dean knows Cas and he knows when Cas is hiding the full truth. Which causes him to think Cas is lying about something important he should know about and he's determined to find out what.
What Cas doesn't want Dean to know is that maybe back in 2009 he started letting his vessel age to conserve grace where he could or so he wouldn't stand out amongst the Winchesters or make himself seem more alien than he already did. But the real reason he let his vessel age (which he's aware of now) is because he wanted grow old alongside the man he loves and this was as close as he could get to the fantasy.











