I think that players 15/16+ should be eligible for open chat. By the time you reach 15 you'd have already matured enough to have a more vast word choice.
Go to your account on the website and select Your Account and then go to Parental control to change that
The person who has the Master Pass is the only one that can change chat options; assuming the OP is 16-ish they are either buying membership with gift cards or a parent/relative is paying for the sub.
In order to change chat options you have to be an adult on file with the account as well as paying for membership with a credit card as verification.
If a parent is in control then it is their decision on whether the minor is mature enough for open chat. It's a legal issue for KI otherwise.
Trust me, 15-16 year-olds have not matured enough to have open chat. Besides, there are Federal laws pertaining to chat in games for those not considered adult age (18+). It's the COPPA law of 1998 (Children's Online Privacy Protection Act).