It is impossible to know the exact age of a blueberry plant growing in a nursery pot of any specific size. This would depend on how long the plant has been growing in the pot; whether it is a "new crop" or an "older crop." That said, below is the approximate age of blueberry plants growing in various size nursery pots.
1 Gallon Pot - 12 to 18 months
2 Gallon Pot - 24 to 30 months
3 Gallon Pot - 36 to 48 months
At what age will a blueberry plant bloom and produce berries?
Blueberry plants will produce flowers and fruit at a very young age. Even well-rooted plants growing in a 1 gallon size pot will often produce flowers and fruit. That said, newly transplanted blueberry bushes prefer some time to establish themselves in their new home (your garden) before producing a strong harvest. It's not absolutely necessary, but to grow a blueberry shrub that will produce larger yields in the future, it's a good idea to pinch the flowers off the plant for the first two years. If you just must have some blueberries the first year after planting, you could pinch only half the flowers off. Pinching some or all of the flowers off allows the plant to use its available energy to establish a heavier root system and foliage, which in turn produces a heavier yielding plant down the road.