As gov.uk states:
You can use the EU/EEA channel to get your passport or identity card checked - this is usually faster than the other channels.
You can use automatic ePassport gates at some airports if your passport has a ‘chip’ on it and you’re over 18.
These gates use facial recognition technology to check your identity against the photo in your passport.
Some airlines (or maybe all, I do not know) allow people the age of 16 or over to travel alone.
What practical reason is there for having the 18+ age requirement to use these ePassport gates?