Now, excuse me if anything I say/suggest has already been stated. I would read over them all but I feel I'd forget my suggestions..

Alright! right.. Lets see. As the comment right above mine says Aristocratic, but I thinking also EGL and just more classic styles of Lolita. Sweet is obviously very childish but I think Lolita can be plenty mature if you get the more regal designs with lower neck-lines and fitted waists, and of course long skirts. Lolita is Sort of like Victorian dolly, child like style so dressing in a more mature version of it I'd imagine just wearing same time-era just the adult styles? Victorian, Edwardian, ect. I think it'd all be safe.. I'll link to some things I believe would work for the cause. It'll mostly be F+F Links and some wonderous MM but I am just showing to get my idea more clear across.

[X] - [X] - [X] - [X] 

I feel when it comes to being a more mature styled Lolita, you start looking more for in the design, versus the pattern. And Going for more muted, dark/rich colours instead of bright airy ones. And of course, going for more fitted waists, corsets, longer skirts, and lower necks and longer sleeves. Wearing tights and stockings instead of knee socks, and wearing 2-3 inch heels with more simple designs, or boots. (Not tea party shoes, or mary janes with lots of straps and bows) I always prefered the more mature end of the Lolita styles myself, but some get stuck in thinking Lolita is always forever sweet. . . Lord I rambled on a bunch. Sorry!!

If that day comes, I would go with a more simplified while neat design, while still show my fancy for lolita at some details, or accessory, such as bag.

I don't know how tall you are, but if you're short then Innocent World often releases dresses that are longer than usual Loli length. (So they'd be tea-length.)

Example: