You're still young!
But I would recommend that you be sure that you attain BPS accreditation in your course - I'm not sure if joint honors with criminology provides this? Furthermore, I would probably go for psychology alone (more limited options for criminology?) due to the lack of 'specificity' of psychology - would you be better off doing only Psych than mixing both, and specialising in criminology through degree options (be sure that the uni course has such options).
Experience in settings you may want to work in is essential, and I would do it as soon as possible.
I was and to some point still am considering doing medicine as a graduate, and I feel that I would still be happy going back and 'starting afresh' - as long as you feel you will be relatively happy in doing so I see no issue. Relative comparison of ages isn't particularly important, I feel.
Note, also due to more and more people doing degree's, be prepared to have to do further study to be competitive in your applications. Hope is not the case, though. Either way, you will require a 'good' degree of a 2.1 or above.