Congratulations 
Testing - ohhhhhhhhhn such a personal one and only you and DH can decide this, you'll know what's right in your heart at the end of the day. There are lots of threads on this to read through but meanwhile, it depends what you would do with the results tbh. Amnios only pick out chromosome abnormalities, Down's syndrome being the most common of those. It's also DS that I know about so with regard to DS - would you be happy and love your child whether he or she had Down's syndrome or not. Are you worried about other chromose abnormalities?
Re the age - more babies are born to women who are younger, but that's cos more younger women are having babies. The women i know whose young children have Down's syndrome are all late 20's early 30's. I also know a 21 yr old girl whose son has DS.
With my DD1 I was 30, had no tests and a really wonderful, relaxing, healthy happy pregnancy. DD1 was born with DS and I thank god that I didn't know before hand. I dodn't wish she had DS but I'm over the moon she's my DD, I love her and am so pleased she's here. It's not all easy but it's all good at the end of the day. She gives everyone so much pleasure and the world is most definatley a better place for her being in it.
DD2 - everyone fussed and panicked me during my pregnancy. It was hell. Had tests, was the worst time of my life, honestly - hell. Was told things like 'it doesn't look good' and so on. Tests came back ok and sure enough she seems to be without any special needs so far. I just wish that i hadn't had to go through the hell of testing and if it hadn't been for DD1 having DS everyone wpuld have just left me alone and I'd have had a wonderful pregnancy from the start. Anyway, what's done is done and it all turned out ok and I didn't do any damage and she's here with us now.
So that's my story, well that part of it.
I hope you reach the decision that is right for you and that whatever happens it all turns out oik for you.