@lavinia , I am personally dubious of your claim that there might be a decline in young people's interest in STEM, mainly because you are presuming that people interested in STEM would necessarily come to PF first, as opposed to other sites on the Internet.
That being said, as
@Frabjous has pointed out, it is difficult to assess the speculation you made without statistics on the following:
1. The posting history over the past several years (which, according to
@Vanadium 50 as per his post #25, is available in raw form in terms of number of messages, and someone will have to actually count the number of posts).
2, The age distribution of the posters on the forums (which as far as I know is not consistently available).