I think I gave it to mine when they were about 9m.
Thereis alot of "stuff" about Quorn around - whether it's safe at all, never mind for babies but I reckoned (and my HV agreed) that since they weren't going to eat it every day and not eat loads of it, it couldn't do them any harm if, indeed, it does anyone any harm anyway!
The manufacturers of Quorn recommend that it is not introduced into a childs diet before the age of two years. Textured vegetable protein may be difficult for young babies to digest, and its salt content must be watched.
It's not the same as mushrooms. Not to get into a big argument but it wasn't approved by Food & Drug Administration in the US for ages because test group had migraines etc. I know it's been available here for a long time though. There are some old threads with better information.