I've never heard of any restrictions on feeding your baby mushrooms.  If they're eating other veges, they should be OK with mushrooms.  My daughter loves baby mushrooms, sliced and cooked in butter - she eats them in her fingers.  If she's really hungry she'll even eat them raw.

here is a site i use heaps on what age to give bub food or spices www.wholesomebabyfood.com

I don't remember there being a restriction on mushrooms, but here is a link to BabyCentre (UK) website, but there are sites for each country - so if you want to ask there for further peace of mind ...

Peace
EF.x

not sure what age they are supposed to ahve them but i use them in cooking too

My little man is about 7 1/2 months and he has been eating a mushroom/pasta  blend  that  I make for him since he was 6 months old. He loves it and is thriving.