Paint like coatings have been around for a long time claiming to extend the life of your roof for years. Until the late 90's they actually didn't do much more than camouflage a dying leaking roof -- the problem being that things move up there from full sunlight to cold nights, from hot summers to cold winters. The roof lives in the toughest environment of the whole house.
Finally there are a number of products, usually elastomeric (rubber) based that can in fact extend the life of an existing roof. Probably the only way to tell one from the other, as many come and go, is to get references of jobs similar to yours that were done a number of years ago. Talk to these people about real performance and was the cost worth the time gained before having to re-do the roof anyway.
These coatings are best developed for metal roofs such as the tops of RV vehicles but some will actually bridge all the overlaps of a shingle roof and create one continuous membrane. Click here for more details on HOW DO YOU PAINT A METAL ROOF.
One caution with shingle roofs is to be sure that there is good ATTIC VENTILATION as we are now potentially trapping moisture that may have previously found its way through the roof itself.
Liquid membranes over old roofing are not substitutes for a well designed and well built roof that does not relay on waterproofing to shed the water away from the house. They only buy you time to extend the life of an old roof -- but not all the products on the market will even do that very well.