Keeping out of the sun is one of the best ways of keeping your skin looking young. You can tell this very easily by comparing a part of your body that is usually covered with clothes with a bit that is exposed all the time.
If you have to go out in the sun a product with a sunscreen is not a bad idea. I don't think that there is any need to have a very high SPF value - even SPF 2 blocks a lot of light. And high SPF creams tend to be very greasy which discourages you from reapplying.
The SPF simply measures how the skin reacts to light in the short run. It isn't directly related to its anti-ageing properties.
What is not always appreciated, even by my colleagues formulating products, is that oils like olive oil can also protect against the sun. Even a simple moisturiser without any SPF will give some protection. And as fishie says, a foundation gives a lot of protection too. I don't agree that the SPF number itself is the most important thing to look for. I would suggest find a moisturiser that suits you and use it regularly, and if it has a high SPF all well and good but don't worry too much if it hasn't.