My neice and her boyfriend are 24, and they have a 3 year old daughter (my great niece). They are lovely people, my niece is a care assistant in a residential care home for the elderly, voted "favourite carer" by the residents and staff, and her boyfriend is a gardner.
But they are so, so childish. They both sound like children talking iyswim. My niece always has been childish, but she still dresses very bizarrely (loads of bright colours that clash, plastic jewellery, hair dyed lilac in bunches), she gets excited about everything and is always clapping her hands, dancing, I just feel uncomfortable watching! And her boyfriend only talks to her and his daughter, he has barely uttered a word to me! He also acts immaturely and gets excited about everything, and has tattoos of cartoon mermaids and kings and queens all up his arms 
The other day we visited them (we live 3 hours away) and when we arrived all 3 of them were sat around a plastic table in the lounge colouring. No adult conversation going on, whenever we asked them anything it was always a quick answer. My niece kept coming up with strange conversations too, such as "Maybe the earth is flat and astronauts are lying to us!" Their lounge decor was very strange as well, they both seem to have this obsession with the ocean and so the walls were full of pictures of sharks, dolphins and jellyfish, as well the many tanks of fish they have. When we went out for a walk, all 3 of them were running riot and barging into people and we had to keep apologising to people for them. 
AIBU to be annoyed with their parenting? It is more like three 3 year olds are living together! I am worried about my great niece's development because her parents seem to be stuck in childhood mode. My husband tells me to just leave them be, but it all just seems rather strange to me. They are lovely in their own ways, but I can't help being annoyed with them at the same time!