It's normal for 6 year old boys to drink from a baby bottle.
I used to drink from baby bottles at 6 years old and continued even now as an adult.
I love have having some juice or milk in my baby bottle and even drink baby formula.
I've also seen a 6 year old boy before at the doctors office drinking juice from a baby bottle and he was also wearing a Pampers Baby dry diaper that you could see sticking above his pants.
He seemed really happy and content.
If the 6 year old boy or any other age wants to drink from a baby bottle, they should do so as it's not hurting anything.