How old is too old for a PhD? Are there any age limits (formal, informal, or guidelines) that schools use when deciding to accept someone into a PhD program? I'm most curious about the upper age limits. For example, will most schools accept someone in their 40's? How about their 50's?
I would imagine most institutions would consider it discriminatory to judge on age, and in some places it would just be illegal. The real consideration is always whether the prospective candidate has a reasonable chance of success. Having appropriate education, or a reasonable substitute is of course the main criterion, though demonstrating research-level ability always helps (research publications etc.).

Personally I have seen PhD candidates of all ages. There's some skew to the distribution across disciplines (business and history for example often have higher proportions of older doctoral candidates than, say, mathematics), but this seems to be more a socio-cultural thing than any institutional influence.