My boy has had one since he was born x
I think the general recommendation is that they can have one from one year.
I'd say you're fine, as long as it's not a huge blanket they can get tangled in. My boys both slept with a warmie stuffed cow since pretty much birth. Having it tucked against them calmed them at night and those suckers are like 8-10 lbs so I never worried about them pulling them over their head
At 15 months I think you’re okay! My baby has slept with a comforter since 8 months but I bought a cuski which are nhs recommended as sleep safe