For twenty years, I was a professional home inspector. Every few months, I'd pull up to a house, typically in a retirement community, look at roof and think, "That looks good!" Then I'd look at it a second time and think, "Something's not right." In all these instances, I'd find out the roof had recently been painted - and typically, a less than reputable contractor had sold painting services to people in retirement communities.
These contractors claimed roof paint could make the roof look better, which it did...for a short period of time. It did not add longevity to the roof, as some had been promised.
Roof paint is advisable only on flat roofs, and for reflective purposes. Some flat roofs, especially those made of layers of built-up tar, use paint called fibrous aluminum as a top coat. Its purpose is to reflect UV rays. Typically, though, on roofs that aren't flat, shingles should never be painted. They aren't designed to be painted. You can ensure a long roof life in other ways, though - like properly installing and properly ventilating your attic. Overheating speeds up the deterioration of a roof, because the hotter the asphalt gets, the more protective oils come out of them, and they become less water-resistant. Painting shingles is unnecessary and often associated with rip-off contractors. And if you're thinking of replacing your roof: Unless your roof is badly deteriorated - and it doesn't sound like it is - you're not going to get the return on investment that makes replacing it worthwhile. Instead, if you plan to sell anytime soon, I recommend disclosing the age of the roof to prospective buyers, thereby taking it off the table for negotiations. Then, use that money you would've spent on the new roof on storage, organization, paint, and other corrective efforts that make your house attractive for sale - and will provide financial returns.