You can't know if it's sexism, ageism, due to speech mannerism, region of birth, college you attended, etc... anything would be pure speculation, and it's really just a distraction.
If you feel treated unfairly -- emphasis on feel -- or if you feel your well-being or behavior are affected -- emphasis on feel -- then you need to focus on yourself and identify which micro-behaviors make you feel this way, then find a way to either change them or accept them.
Grant me the serenity to accept the things I cannot change, the courage to change the things I can and the wisdom to know the difference.
-- Reinhold Niebuhr
As an outsider it's hard to know whether you believe things can change, and what the best way to enact that change is:
- It could be talking things out with this colleague: they may, after all, not realize themselves that they treat you differently, or believe it is for your own good.
- It could be raising the issue with a colleague you trust, perhaps a senior figure in the company, or with your manager. They may help you either change or accept things.
- I would not advise pulling in HR just yet, not before trying other avenues first, as it's the "ultimate" hammer and may significantly degrade your relationship with not only this colleague but also others -- as unfair as that may feel.
The one thing I can advise you about is how to talk about it. You should avoid accusing, and putting anyone on the defensive, and therefore the best way to frame the discussion is to talk about the one thing you know for certain: how you feel.
A framework for this is SBI: Situation, Behavior, Impact. When raising the issue, no matter who the interlocutor is, present the 3 points, in order.
- Situation: recall one specific situation. Don't be vague.
- Behavior: describe the specific behavior which you found problematic. Yes, it means identifying it first, as precisely as possible.
- Impact: describe the impact on you. How it makes you feel.
If there are other situations, and other behaviors, rinse and repeat. Always be specific, or as specific as you can.
From then on, you've presented the problem, it's time for an open discussion with whoever you are talking to about potential solutions. They may suggest you are oversensitive. They may be right... and you may want to work on yourself. Or you may find this advice insensitive and unhelpful. Talk it out with them if possible. Keep an open mind. Relationship solutions are often middle of the road kind of things.
If you still don't feel well after this discussion, mull it over and pick another time, or another person. Even if it doesn't immediately solve your issue, hopefully it will give you material to reflect on, be it clues, better way to present the problems, more specific actions or feelings, etc... and progressively you'll work it out.