If it was me I'd opt for Soller, I love the town and the port, it's one of the nicest on the island. As for it being geared towards an older crowd I can't say. It's certainly not Magaluf but equally it isn't retirement territory either. We don't stay in Soller because we have a Marriott timeshare in Llucmajor in the south west of the island but we often visit when we do stay. The Tramantuna mountains are my favourite part of the island which is where Soller is situated and also the towns of Valldemossa and Deia. We also like the Jardines de Alfabia in nearby Bunyola (www.jardinesdealfabia.com/en/) which can be visited prior to the tunnel that leads into the Tramuntana Mountains. I don't usually offer restaurant recommendations as peoples tastes are different so it is always completely subjective however..... Casa de Sa Miranda in Valldemossa is very good plus the views are amazing. You'll probably need to book if going at a busy time as they're recommended in the Lonely Planet so they receive a lot of custom.
Arta is another lovely old town which is dominated by a fortress at the top of a hill and inside is an old church. The views from the fortress are impressive and the town is full of narrow cobbled streets with museums, independent shops and plenty of restaurants. Nearby are the Caves of Arta, hugely impressive caves full of stalactites and stalagmites which can be viewed on guided tours. I would suggest visiting the caves in the morning and drive to the town of Arta to climb up to the fortress and then have lunch in the town.
Palma is visually very impressive, particularly from the port side and there is so much to see and do there that will easily fill several days. I would suggest the Lonely Planet guide to Mallorca to get a full appreciation of what there is to see and do and tailor your itinerary to what interests you.
I highly recommend a hire car for your stay (I always use rentalcars.com, best prices and reliable companies). The island is very easy to drive around, far less frenetic than the mainland although the roads in and around Palma can be quite busy. If you're planning on staying in Soller bear in mind that the mountain drive can be quite hairy/exciting depending on your view. Most parts of the island can be reached within an hour and a half so day trips are very easy. There are plenty of ports and marinas all around the island and it's very easy to do a port hop with a car (I would concentrate on the eastern side for this).
If you're interested in beaches there are so many to choose from and so many different types. There are wide, sandy beaches with clear, Caribbean like water, sheltered coves, isolated beaches that can only be reached by steep footpaths to unspoilt beaches in a national park, in some cases you're likely to have them all to yourself.
The beauty of the island is that nothing is very far away so if you decided to visit one town and didn't think much of it you can visit another one about 20 minutes away. In my opinion the coastal towns are nicer than the inland ones, however the inland ones are more traditional. There are some incredible markets in the various towns (make sure you obtain a list of what days the markets are on as they all differ) which are an absolute delight if you're a foodie.
One place I would avoid are the Drach Caves. If you want to be rushed through the caves by a tour guide and be ushered into an area of wooden benches overlooking an underwater lake where you'll wait for about an hour if you're really unlucky for the place to fill up whereupon you'll all be serenaded by a trio of musicians as they pass along on a boat. This takes about ten minutes, has no relevance to the caves and is very tedious and frustrating. You are then faced with the scrum of hundreds of people all trying to leave from the narrow exit at once. The Caves of Arta are far better and far less busy.
