The life expectancy of all birds is extremely hard to estimate. It very much depends on how its life is spent. If in a cage, a lovebird would be lucky to see 10. If let out every day for a fly around, or lives in an aviary it could easily reach 20 years of age on a sensible diet.
The vast majority of captive birds die well before 'old age'. They are, by nature, exceedingly adept at hiding ailments so as not to appear an easy target to predators. Add to this the fact that most vets do not know much about birds and it is not surprising that birds usually expire well before due. Birds do not have much body bulk for their size, so by the time that illness is noticed it is usually to late.