Can i use old pentax lens on new digital slr camera?

They are made to accept most of the older lenses.

Pentax KAF mount is the current mount.
KAF2, KAF and KA mount lenses - fully compatible
K-mount lenses - usable with restrictions
S-mount lenses - usable with adapter and restrictions
67/645 lenses - usable with adapter and restrictions

Not too old ...If they are Pentax KAF2, KAF, KA mounts the lenses will work