You can asked to be referred for soft marker scan to be done earlier than 18 - 22 weeks, especially if you have concerns about chromosomal abnormalities.
The triple test is a good indicator at 16 weeks as well.

Although you are young, there are actually many chromosomal problems that are not associated with maternal age, I really don't thinkits fair that people should be told because they are under 35 they are low risk so dont worry about it. With DD I was 29 and had a 1 in 20000 risk, with D angel I was only a year older, yet we lost her due to chromosomal problems.

If you want the test done regardless of the decision you will make (believe me you dont make a decision like that "until" you are in that situation and have the results) just ask your GP for the triple test, and then take it further if you need or want too.