I just acquired an older piano and have no idea when it was last tuned.
Some things to consider about older or long-neglected pianos:
Let the piano acclimate to its new environment for 10-14 days before tuning it.
Many times older pianos have been neglected and may have problems.
If the piano was in a bad location for many years, it may have sustained damage. If the piano was in a very dry location, repairs may be necessary, and there may be tuning stability issues. If it was in a damp location, there may be broken strings, and the piano may need to have mold/mildew removed.
The piano may require extra tuning, above and beyond a normal tuning. It is also likely that the piano will not be at standard pitch.
String breakage occurs sometimes, although rarely, when a piano is tuned for the first time after 10 or 20 years of neglect.
The piano will not usually exhibit normal tuning stability until after the next tuning visit, assuming a commitment is made to regularly