I did Teide last year aged 60; I'm fit, but not a climber as such - I occasionally do a few of the Scottish Munros and run 5 - 10k twice a week only. 20 years ago I did Mont Blanc in the Alps, which I found desperately hard, and didn't acclimatise at all well despite 3 days altitude preparation.
Getting up and down Teide was easy in comparison. I left Puerto de la Cruz about 7.30 in the morning, drove to the little Montana Blanca car park (there is a small notice board to confirm where you are), did the climb of 4,500' or so up past the Altavista quite straightforwardly, back down the same way and was back in Puerto de la Cruz by about 7.00 in the evening. Above the Altavista I took it at more of a plod, and stopped for a wee breather every 5 - 10 minutes; breathing is definitely affected up here, but I didn't have any of the severe headaches, sickness and disorientation I had on Mt Blanc.
Back at the hotel, I felt great, had a shower, then started a beer; I couldn't finish it; went straight to bed, had a poor nights sleep, didn't feel at all well the next day, couldn't eat, and only started getting back to normal the day after that - all absolutely classic altitude after-effect symptoms.
One other minor complication doing it this way is getting the right permit time slot for the summit; I actually arrived at the checkpoint about 1/2 hour before the start of my slot, but the guard let me through after my best pleading look. I have looked at doing Teide in a day from sea level but don't think there's enough daylight - for me anyway; I don't fancy the prospect of stumbling around in a remote, exposed, high altitude volcanic landscape in the dark!