While many banks and building societies refuse to offer residential loans that beyond retirement or age 75, new pension freedoms have prompted one major lender to change its rules.
The Mortgage Work, the buy-to-let arm of Nationwide Building Society, will now offer the under-70s a loan for up to 35 years.
Managing director Henry Jordan confirmed that customers would in theory be able to borrow past their 100th birthday.
"Using buy-to-let as a long-term investment is growing in popularity among people who want to maintain their options and potential sources of income into retirement, and the prospects of the new measures regarding pensions could see even more considering it as an option for their retirement savings.
"Our removal of upper age limits at maturity will ensure customers are offered greater choice and flexibility around the point at which they might sell their property; providing increased peace of mind for their tenants, as well as supporting stability in the wider market."
The move to offer buy-to-let loans to retired customers contrasts the attitude towards residential mortgages, where customers in their 50s are being rejected for loans stretching beyond age 70.
Mark Harris, chief executive of mortgage broker SPF Private Clients, said: "Getting a buy-to-let mortgage should be easier for an older borrower than obtaining a residential loan because the rental income that covers repayments should not fall when the landlord retires, while the employment income used to pay for a residential mortgage does."
He said Nationwide's policy on borrowing ages was the most relaxed among the major lenders. Some smaller, regional building societies had also removed the "maximum age" for borrowing and were lending on a case-by-case basis, he added, while Aldermore, Kent Reliance Building Society and Precise Mortgages all allowed borrowing to age 85.