Age restrictions tend to be around the borrowers ability to afford the loan once the retire.
Many lenders will be reluctant to lend money for a PPOR after you're about 55. They don't want to be repossessing your home, especially if you're in retirement. You can still qualify for a loan, but this would need to be mitigated (lump sum super to pay it off, other assets you can sell off or something similar).
Lending for an IP is quite a bit easier. Lenders recognise that people can sell an IP without affecting their lifestyle or that the rent will cover the cost of the mortgage. Lending for an IP when you're older isn't a big deal if you've put a bit of planning into it.