We all have selected memories and when we are 3-4 whatever our hair looks like we will hold onto that forever and tell people that’s what our hair is, be it black, curly, blonde or thick you will always have this hair as your baseline.
Sorry but this is not an indication of your hair, up too 14-18yrs old your hair is still changing however after that would possibly be more realistic than early. Some ( a large portion ) of children are born with thin hair, which will thicken up over the first 12-18 months it may start out curly and shortly after its cut go straight this is normal.
So sitting in the chair when your 40 saying I don’t know what happened I used to have curly hair ( when you were 3 ) is not a realistic comment.
Texture, density & color changes over time this is normal and is from many factors and 99.9% are out of your control.