I am approaching 70 and I've never had problems focussing, this is probably because I was a microbiologist for 44yr and used a microscope daily. As a microscopist focussing almost becomes something that is done instinctively and the same technique of focussing is the same with a camera. The technique is to focus in and out a few times quickly and with practice the brain instinctively recognises the in-focus point.
For the 'middle aged' whose eyes have deteriorated don't give up hope, at my current age I am lucky in that my eyesight has improved over time from mixed long sight/short sight to only needing glasses for reading.