Age is not the only issue
An older house is not automatically unsuitable. The important questions are how much heat it loses, whether draughts are controlled, whether attic or wall insulation is realistic, and whether radiators can deliver heat at lower temperatures.
A home that has already had sensible fabric upgrades may be a much better candidate than its age suggests.
Example planning sequence
Start with attic insulation and draught checks, then estimate heat pump size and running cost. If the result depends on a very high SCOP, ask the installer to explain exactly how the system will achieve it.
For older homes, radiator upgrades, zoning, controls, and hot water design should be discussed before accepting a quote.