15 years could be considered a long time because a 3 year old toddler would be an 18 year old adult in 15 years and the 3 year old would be graduating high school 15 years later.
And a 1 year old would be 16.
So although 15 years can go by fast it really is a long time since toddlers can grow up to be adults in 15 years time and a lot can happen in 15 years.
A 6 year old child would be a 21 year old adult legal to drink alcohol.
Once the child has gotten to be an adult though the 15 years would seem like they went by fast once the 15 years have passed.