I think you must be doing really well for someone who is passing as someone without mental illness, so that it probably why you believe people don't notice it. However, my bet is that it leaks through, and there is something off about your behavior that you can't pick up on. Just because you're mild-mannered doesn't mean that there isn't something that rubs people the wrong way about your behavior. I've worked with a lot of people who were mild-mannered who behaved strangely and caused people to not like working with them.
My bet is that this is likely the reason why you aren't getting job offers, as opposed to age. Because you are doing so well as someone passing as someone who doesn't have mental illness, you are probably hardcoded to believe that that's not the reason, but I would revisit that belief.
