I work with children - not so much on a day-to-day basis, so I have a store of "cute things they say", and enough contact to keep me positive about children being nice most of the time, and feeling good that I can give them back when I'm done. And I have a two-year-old niece and many of my friends have small children. But I don't see them every day, and I don't share all the celebrations, or have to cope with their difficulties, or mark their milestones.
So when people talk about, say their baby's first birthday, the only comments I have to make are based on experiences as a child, rather than as an adult. So this means I constantly feel like a child, in these conversations. I've talked to other childless adults about this - one, who I'm pretty sure is childless by choice, seems to quite like feeling like a child. But I don't. I don't really like that my acquaintances and friends seem to know more about being an adult than I do. I don't like that I talk to a colleague who is the same age as Mr Spouse and he lumps me together with his pre-teen daughters.
Being an adult seems to mean responsibility - in particular responsibility for children. If you don't have that, it seems you cannot be fully grown up.