I'm 17 and I work as part of weatherspoons, so I can honestly say you should be relatively fine, depending on the branch.
Legally, no one under 18 should be there after 9pm, although that's branch depending as it's 10 for us.
Ordering food and soft drinks is an any age deal, but only alcoholic drinks they check ID.
If you buy around of alcoholic drinks for your table, they will check every id of the person at that table as failure to do that can lose us our jobs.
I hope this is some help