Welcome Nicky.
It's entirely up to you where you draw the line but I'll say this, whatever limit you set there still may be some who slip through the net. I've tried various lower limits and I've found just applying basic common sense works best. Most guys in the 17-23 age bracket never get a booking with me purely because they can't follow a simple procedure of reading website, making a few pertinent enquiries, booking, confirming, turn up without a massive fuss, becoming demanding or pushy. All of which has me running a mile and not letting them any further. I have however had the odd 17-23 year old phone and make a straightforward booking, turn up and all goes absolutely fine. I'll admit to sometimes feeling a bit wierd with men almost 20 years younger than myself, but I manage to cope with it somehow. If however you really are not comfy with someone under a certain age, just make it clear on your profiles and website. You might want to mention it when the booking is being made, but as I say some might purposely lie and it's not always that easy to tell visually either.
Good luck with whatever you do.