A criminal record doesn't necessarily stop you getting the job - if you don't declare a conviction and are found out, then you won't get the job but if you do declare it, a risk assessment is done on both the person and the role, and issues are addressed from there, e.g. supervision at all times, no access to records or cash etc - it all depends on the conviction, the job and the person. If you had a petty conviction at the age of 17 and are now 30+ it could've just been childhood mischief that you've grown out of, so these things are taken into account.