Like most residential mortgages, most lenders will accept applicants on the mortgage paperwork up to around age 80 - although some lenders have no age cap.
It is vital to consult an independent broker before applying for a JBSP mortgage; since your joint applicant's age may influence which lenders you should apply to. For example, if a parent is nearing retirement, you might find that a lender will only offer a mortgage with a short term, which ends before they reach a specific age.
You can also apply for a joint borrower mortgage for a buy to let investment property.