Bro. 18 yr old is not a child by any stretch of imagination. Accor charges vary based on the original room size you've booked and if rollaway beds can be accommodated. For a 18y old there are places where they would not allow more than 2 adults as per local laws (like Japan) and you'll have to book another room. However if room size is large, you would invariably have to pay for the extra bed. Unlikely you can avoid this at that age. Maybe till 8yrs etc. they may even consider.