Wow man I had an extremely similar situation, im 20 and my "older bro" flatmate is 25. I think it just naturally happens when you end up buddies, live together, and theres an age difference. We'd been really good buddies since I moved in, after like a year he did fall into this attitude of being the eldest wisest. I didnt confront him, im not the "we need to talk type". I didnt make a show of it but I started shrugging him off when he said anything that seemed to figure me out as younger and less mature. I started hanging out with my friends down the street and doing my own thing without him.

When he saw I liked him but didn't look up to him everything went back to normal again