If your parents really are elderly, maybe Luzern would be better, as it has a nice town and lakeside you can wander around with frequent sit down opportunities.
How long can they walk for? 10 minutes, ½ hour, 90 minutes . . .?
For Titlis there is a ~10 minute walk from the station at Engelberg (town at the foot of Titlis), to the cable car on the edge of the town.
As caldbunch says, Luzern is good for Titlis (or Rigi or Pilatus). Interlaken (or Lauterbrunnen/Wengen/Grindelwald) for Jungfrau.
At Luzern you have the lake boats, leaving from the quayside in the town centre in front of the station.
You can do a circular trip where you get a boat round to the far side of Pilatus, funicular up to the summit, cable car down the other side and then a short bus ride back to Luzern. Details here: https://www.pilatus.ch/en/discover/golden-round-trip/
Click here for a 360° panorama of Luzern: http://tour.luzern.com/360/en/
The old town is to the right of the bridge, along the river. the station and boat quay is to the left of the bridge. Above and slightly to the right of the station, the mountain with cloud at the top is Pilatus. Rigi and Titlis are further away.