I'm sure in that part she said in a affectionate way or jokingly (Otherwise that wouldn't be very healthy for their relationship tbh) because when Miles talks about his maturity beyond the age he is during the tower scene watch, she laughs and keeps telling him how lovely it is to be with him. For the rest of the movie, he treats him in a normal way.
Gwen said "I used to play with these when I was younger, too" like she was 15 years older than Miles, not 15 months 😂