The truth is, noone really knows the lifespan of solar PV, but it's certainly in the decades. How many is anyone's guess...
Dream on Andy, after 20 odd years in the solar industry I have left a bit disillusioned. You see, all the panels I put in 10 - 15 odd years ago are failing and quite a few I have fitted in the last five years are on the way out. Some panels may last 25 years plus, but not all and in the case of the grid connect systems that are everywhere, the failure of a single panel will stop the entire array working until the faulty panel is replaced. Replacements are hard to come by, warranty service is abysmal, roofs are leaking and the whole shebang was/is sponsored by taxpayers. On top of that grid feed inverters are not as reliable as promised .... Further to that, the average grid feed punter would have no idea if their system was even working correctly.
I also agree with Lodown, output degradation starts when these things first see sun. This is why the 20 year guarantees you talk about are output related and at 10 years, the output guarantee is only 80%. Further tho this most solar manufacturers who offer 20 plus years of warranty haven't even been in the business 5 years!
Don't get me wrong though, I live with solar power, love making my own electricity and wouldn't take a grid connection if it was offered free. Unfortunately the unreliability and warranty issues have made long term solar folk slightly peeved.
The big problem is that electricity consumption per person is rising around 10% per annum and most people have absolutely no idea how much electricity they currently consume. Do you?