Candidates would have to include:
- DC-3s
- Beech 18s
- Grumman Goose
- Ju 52/3m (assuming sightseeing flights by JU-Air count)
I'm sure there are pre-WW2 or early WW2 examples of each of the above still in service. So it would be a question of looking at individual airframes to see which is in fact the oldest.
Lots of DC-3s in service, for example, may be late WW2 build, and so potentially "younger" than other airframes. Likewise, although the Beech 18 design is very old, production actually went on for quite a while, so a particular airframe need not be that old.