As an aside, I don't think it is helpful (or safe) for a minor to hide their age in their 1x1 interest checks to potentially get more activity, as some people in this thread have advocated for. The actual number may not be necessary, but a heads up like, "I am under the age of 18" is appreciated β and it's not just an internet security, cover-your-ass thing.
In my opinion, hiding it would be disrespectful to potential partners who may have personal boundaries about not roleplaying with minors. They may not want to roleplay adult themes (not necessarily sexual things, but topics like addiction, violence, other dark story elements) with someone under the age of 18. That is fine β it is their prerogative to have limits on who they want to tell stories with.
They also may not want to roleplay with someone under the age of 18, period. That's also fine. They are allowed to want to write with someone who is on equal footing, with similar life experience and maturity, regardless of topic or genre. Considering that roleplaying is a social hobby that often leads to internet friendships that last long after a thread dies, it is not at all wrong that people should want those potential friendships to be with people in their own age bracket.
Calling it age discrimination is ignoring the potentially-valid feelings of the person on the other side of the screen. Key word being person. They would be fully within their rights to drop anyway if they do find out you're underage, so it may save everyone time and effort to be upfront about it.
Trying to mislead someone into roleplaying with someone they would otherwise be uncomfortable writing with (if they knew the full circumstances) might be something a lot of us did as shitty teenagers on the internet, but... was it good of us to do that? Looking back, probably not.