I appreciate that the authors didn't go too far into speculation territory, but this being the internet I'll do it anyway.

The survey only explores whether people feel like they became the person they want to be, but it would be great to know many of them actually act in a way consistent with their ideal self. Perhaps it's not that older people feel more comfortable in who they became but rather that, after 50+ years of working at it, they have finally nailed their target and there's not much left to do.