To everyone saying Aria vecomes a GRANNY
I think many people are exagerating the difference between the two step-sisters.
Sure, Aria is pretty much grown up, and sure it looks kinda weird that happening to like 14 year old girl, but even though, it's not like they had started at age 10 (Aria is 14 and Mielle 13). They started middle way the age of their growth. If her growth happened little by litle over, lets say four to five months, it won't be too weird.
At most I'd give her like 17 years in her current look.
Looking at it from real world experience:
I've known a girl that while being 20-ish year old everyone asked her for her ID while buying alcohol (18+ stuff) and were surprised how young she looks (generally due to height under the average)
Another one that while she was 14 year old, she had looks of 16-ish and... quite the huge boobs as well... (Honestly a bit too big for my taste, lol).
While it is true that Aria while going to casino looks old enough to be an aunt it is most probably to magic of makeup to make her look mature, old enough and most of all a tad different that the random kid she was coming with aka: the maid-thing.
So in my opinion it is magic of makeup and just time progression that we seem to lack. Remember that manhwa cuts parts and doesn't directly show the flow of time like with the wig from the last chapter (that was used while in the novel apparently) so it is only natural that viewers are being confused while manhwa (while doing actually good job) just tries to not overextend the story too much.
Aria won't become wrinkled granny, at least she got a couple dozen-thousands of use before that.
I guess she used only like 3 years of her life and now she got the money so it means that she wont be using it that many times, since it can go back only 5 minutes back, it's not like she can reverse her month worth of investments with it and she wants to be prudent with it as well (even before entering casino, but decides earning money is a must).