You just need some form of ID to send / receive WU, so any form of drivers license will suffice. In my country, you can drive @ 15. So you can use your license from age 15.
Usually on the WU site, www.westernunion.com they will have a FAQ, or a helpline that you can call, toll free from your country.
Depending on what country you are in, there are different age limits/restrictions etc.
For more information, try going to Western Union costumer support, i'm sure they can help you. https://thewesternunion.custhelp.com/app/home/?pid=usMenuHelp
Good luck finding what you need.