Nah, not at all, I am just a preggers ole fart hehehehe.
Amniocentisis is when they put a needle in the uterus and take out some fluid and cells - analyse it - they look at the cells and can tell for certain if bubs has any chromosomal problems. There is a risk of miscarriage - can't remember stats though - so only recommended if screening tests have come back high risk or if you have other children with genetic differences. It happens around 16 weeks usually.
So, for your age - it is up to you. You are not considered high risk age wise - but you don't have to be "more mature" to have a baby with downs either :)
Downs screening is not routine - it is done around 12 week mark for reason Tanstar said, to give couples options I suppose.
If it won't make a difference to your family and you wouldn't terminate etc - then ask yourself why you would have it done - very personal decision.
Hope that helps a bit more ??...