At what point is someone considered too old (in terms of experience) for an Assistant Professor position?
No sane committee is ever going to turn down a candidate solely for being 'too experienced', and it is rarely meaningful to reduce someone's CV and accumulated experience to just a few numbers. However, this comes with some qualifications:

If someone is reaching career milestones (e.g. first Asst. Prof. job) much later than the norm, a committee might question why, and whether this is a sign that future development will be slow.
If someone is applying for a job that is 'too junior' for them, committees might be concerned that either (i) they're not really a serious applicant, or (ii) that they'll quit as soon as something better comes along.
By definition, if someone has more experience, then the committee has more information on which to base a decision. Junior candidates often have to be assessed in terms of 'potential for success' rather than 'evidence of success'. This can count for or against the more experienced applicant: some committees may prefer to gamble on a rising star, others may prefer the security of an established candidate.
Hiring decisions (just as everything else) are influenced by many external factors. Everything from financial considerations (experienced candidate may cost more) through to government policy (how candidate's metrics would influence the university's ranking and hence income) may help sway a decision for or against a particular candidate.