Anyone know how old you have to be to apply for a guard role?
As per what
@craigybagel said, although it's 18, it's extremely rare for a person that young to be recruited as a guard.
For safety-critical roles, they will be looking at your experience and asking you about this in interview. Most young (early-20s) guards start off in ticket office, on gateline, etc. and then go for an internal vacancy after being in that role for a few years.
If you can take and fail guards assessments an unlimited number of times (I don't know about them - others will), I'd say that you might as well apply as it will be at least good practise, but just be aware that you would be unlikely to be successful. If you can only fail the guards assessments x number of times (like drivers assessments), I wouldn't want to risk failing them when you're at such a young age that you wouldn't be likely to get the role even if you passed.
Hope this is helpful. Sorry if it's not really what you wanted to hear.