Promise rings in mid 30s

Tl;dr: what are thoughts on promise rings in 30s. Cute? Or immature and sign of being uncommitted?

My (34f) bf (35m) have been together 3 years. Blended our families. He keeps bugging what I want for Xmas. But I never really know what I want. Also don't like to tell ppl what I want as a gift. I am a very much the thought that counts type of person and will love anything he gets me rather than telling him what to get me ya know? I went to do some shopping on the computer and noticed he's been looking at promise rings. While I'd love a just because ring. Aren't promise rings for kids not old enough to get married or men uncommitted like an IOU engagement ring. Or am I thinking to deep. I would love a peice of jewelry from him. And didn't think much at first but then I remembered a group of women telling me this before when another received one.