It happened to me and I asked the client to show me a valid ID card. I explained him that it's a very serious concern and that I needed to be sure he was old enough to give consent and he did not mind showing his ID.
I must have had a stupefied expression and I could not help but laugh. In fact, personally it was something that did not bother me but there was definitely a big discomfort afterwards. Clearly it was the funniest moment of my career ��