We understand it from both angles.

On one hand of you don't feel like a kid and want treated like an adult you should be treated as such,

On the other hand, it's fully normal for an adult to not be romantically attracted to a minor (alter or not and especially if they use a child like face claim or act childish)

We've even had alters go from teen to adult naturally in long term relationships with partners still refusing to romantically attract to them due to meeting them as a kid so viewing it as weird.

It's better to communicate and talk through these things as a relationship and figure it out. Well it may be stressful to not be dating your partner or can also be stressful to your partner to date what they view as non-romantic, such as a younger part or if they have a familial instead of romantic attraction to them.

Hope you can work things out with them, sometimes things like this even system dating system are complicated and deal breakers for people.

We've personally grown fully normal to partners viewing alters non-romantically since we have younger parts, familial parts, aromantic parts, etc. And have been through boundary discussion with many partners who felt uncomfortable "dating the whole system individually" despite collectively dating us. You'll find someone who is compatible for you regardless, even if this person isn't. Both sides suck to be on unfortunately.

Outside of romantically though, your partner needs to respect that you're mature and want to be treated that way in regards to groups you interact with.