They won't recruit Police Officers under a certain age no, but you can still get a job within the Police doing other stuff.
I applied for 9 jobs today alone. Doubt I'll even get a single interview out of any of them.
I applied for the Police soon as i turned 18, got short listed to Manchester Acadamy. The selection process, Exams, Role plays and more exams isn't easy. Then if somehow you pass those you go to an interview to see if you will fit criteria.
I know for a fact if i nailed the SP, the interview they would of said the 'no life skills' scenario. Although it was an eye opener and gain a heap of experience to say the least.
Just turned 20 and now in the ambulance service, absolutely love it.
However like kyn says, got to stand out from the rest because the way the economy is at the moment.....
Hardest part for any job within the public services - police, fire or ambulance especially is not the tests but to get to the interview stage.