Every roof is different depending on weather and shingles and installation.
Even though shingle manufacturers say that their shingles should last 20-39 years, this doesnt take into account the crazy Texas weather. The long hot summers, hail, wind and other factors can bring the life of your roof to a early end.
Proper ventilation can help the roof last longer.
If you are concerned that your roof my be reaching its life span have a qualified roofing expert do an inspection.