Morally, I think it's more that you have severe doubts = it would be difficult to give them a good service.
If you, or anyone else, decide to have clients like this, it'd be a good idea to get proof of age. If you have a reasonable belief that they're above the age of consent, that's enough legally, but you probably don't want to feature in an 'this escort took money for sex from my 15 year old son' or similar story.