I'm no expert so what I'm gonna tell you is based on what I've read and personal experience.
There are a number of ways to tell that a shingle roof needs replacing (but not limited to what I have to say). Yes, the age is one thing to consider. BUT, a 20 year shingle could deteriorate sooner if, say, it faces South and gets TONS of sun. A lot of direct sun will break down the singles and shorten their life by a couple of years. On the same token, if you're roof faces East/West, and you get decent shade, your roof's life may be extended by a couple of years. You can also tell if your roof's time is up by making a random inspection up close. If the edges are breaking off because they're dry and brittle, it's time to get a new roof. If they're chipping a little on the corners, then it's time is up in a year or 2. At the same time, if your shingles are soft in the sun, then you've got some time yet. Of course, no matter what the condition of the roof is, if you're leaking water all over the underside of the roof, you're in for bigger problems!
In short, do a random check of parts of the roof that are in the sun as well as parts that aren't. Look for chipped/broken corners. AT the same time, if you plan on being in the house for 20-30 years, get the roof stripped off to the sheathing and have new tar paper and a 30 year fiberglass shingle installed. This way, while you're still working (and I assume you are), you can more afford to have all this work done and in 30 years, when you're retired in this same home, the next installation of shingles can be put right over top of the ones done 'today'. In most jurisdictions, you can only have 2 layers of shingles on a roof. Then, on the other hand, if you're only planning on being in your home for a few more years, have a new 20 year roof put on over the current one (if you'd like to save a little $$$). Drip edge should be installed and your gutters should be inspected.
Not much else to help and I'll be watching to see what others have to say. My best to ya and hope this helps.