Without context (which I don't have), it sort of sounds like Elise wants to be referred as an adult and Leo is making jabs at her.

Though an interesting trivia I found in TvTropes:
"In Medieval/Renaissance times, noblewomen were considered adults and marriageable at around age 13-14."

Gotta disagree with OP on this one

Look at the wording of the Japanese, carefully:
"She's STILL such a brat"
If the word "still" wasn't there in the Japanese, maybe you might have a point. As it stands, no, she's older than she's acting like in both versions