I have an older brother I've always got on well with. Even though we're both grown up, there's still a tendency for him to play the role of older brother with me. It's bearable, but quite irritating. So a couple of years ago, I raised the subject diplomatically and asked if he could drop the big brother act. He got the message and said he would do his best. A few weeks later, he explained patiently to me that although he was trying, it was very difficult because I was still doing the kid brother routine. Do you know, I hadn't even noticed, but he was quite right.
When you've spent 18 years in a role, it's so easy to sink back into it. Try as we might, people get pigeonholed in families: the clever one, the absent-minded one, the shy one, the bossy one, the unreliable one, and so on. These roles play off one another -- you're the clever one, or the bossy one, in relation to your siblings. After you leave home, it may well be that that you're actually not so clever, or silly, or funny, or considerate in relation to everyone else.
So you find your own level in the big wide world, but whenever you're around the family, they expect you to revert to being funny, bossy, pompous, or laid back. Because they expect it, you do it. The role is so natural you slip into it without thinking. Meanwhile you're expecting them to be whatever-it-is they always were, and they're obligingly cooperating.
None of which should be a problem, except that it just is. It's frustrating when your family treats you like they always did, as though you hadn't grown up. In fact sometimes it goes well beyond frustrating, to the point where it can cause real trouble.
So if you want them to drop the bossy act, you need to stop acting dopey. If you want them to take you seriously, stop playing the joker. If you're fed up being patronized, stop behaving like a kid. It will take them months or years to notice and respond -- it won't happen overnight -- but in the end they'll learn to judge you by your behavior now, not the way you used to be years ago.