I wouldnt worry, although that's easy to say. If you're concerned, get the 12 week and 20 week scans as they will tell you some things. Also I had DD1 at 24, and DD2 at 32, and it was exactly the same. Now Im 35 pregnant with DD3. I would say I had more scares with the first than any other. Also my step sister had a very ill baby (that didn't make it to full term) at 23 and that was after a healthy baby, and now she's older and has had 2 more healthy babies, so forget the age thing. Just enjoy your pregnancy.
As for things like autism, I don't think there's much you can do about it, so I would suggest not worrying if possible! Sometimes they say worry is worse than anything. Good luck and enjoy xo