UPDATED: NOVEMBER 10TH 2018
Most clubs in Liverpool require you to be 18 years of age. The places that require a 21 photo id are generally the trendy bars and clubs, even then, the doormen will just say it's 21 if they don't like you. Ninety percent of Liverpool's venues the age limit is 18.
Liverpool has so many bars and clubs about Liverpool, if someone says you can't come in, you just go somewhere else. Lots of good student bars and clubs about Liverpool who will gladly welcome your business — younger crowds populate around the Concert Square area. Probably best to head here first.
In short, you'll be fine! And if you all have ID (which we recommend if your visiting Liverpool for the first time), maybe student cards.... its a WIN WIN!