If you're making genuine worthwhile posts with considered, relatively detailed points and people are being negative towards you simply because you're fairly new to the forum, that's poor on their part. Unfortunately in such a large forum you're bound to get some people with that elitist mentality.
The way I see it is that if you contribute good, worthwhile posts to the forum, your join date is completely irrelevant.
If it's to do with your postmaxxing, some people just get annoyed by what they see as spamming. Personally, I have nothing against postmaxxers who contribute decent, worthwhile content to the forum as opposed to constantly shitposting to crank up their post count.
It's hard to tell without specific examples why people are being negative towards you.