When is it time to replace the roof?
It depends on what your goal is.  If the goal is to never have water enter your house in any way then probably what you need to do is get one of the shingles that fell off and see what its lifetime rating is.  Most are 15-20 years.  If you are near that point and shingles are coming off then I would think about it.  

However shingles falling off doesn't necessarily mean you need a new roof.  It could have been a bad install on that part of the roof - or everything.  

What you read is pretty true though except for the vegetation part.  If you have tree coverage over your house you might get moss with a 2 year old roof.  The curled, cracked, badly warped shingles... this is extreme and should be replaced.

If you want to wait to the last second, then you need to venture to your attic a lot - after big rains.  Most of the time a roof will show signs of moisture before it starts letting in a lot of water - unless your roof is pretty flat.  If you don't want to get up in the attic to do this then it is purely a guess/luck.

Last thing...  How long do you plan on being in the house?  If it is less than 10 years.  Do you plan on replacing before reselling?  If yes and yes.  Then just replace it and enjoy the roof.