From a google.com search, the general consensus is that they bloom at 3-4 years of age.

Getting a smaller sized Jacaranda to bloom is more a function of suitable climate and heat/sun, as they can easily be found blooming as young nursery plants at only 5 to 6 feet tall here in California. On the other hand, without the right conditions of sufficient warmth and length of growing season, they can also go years without blooming at all here locally, especially if they get cold winds and fog off the bay. They also seem to bloom best where they go a bit drier in winter, but do handle our winter rains without problems in the San Francisco area. Early warm springs also suit them, as in southern California, where they can bloom as early as May. Typically they bloom later here in the SF Bay Area, mostly in June/July or sometimes even not until August when the summer has been on the cool side.

Yep-I've had them bloom in five gallon pots-but mine were selected nursery strains,not seedlings. Funny thing is-when you plant those 5 gallon sized plants that are about 4-5 ' tall,they take three more years to bloom establishing a ground root system.
My neighbor across the street one day had little itty bitty Jacaranda seedlings he must have dug up around a friends tree..and in two years they are a bushy 5+ feet . FAST growing.
btw,They bloom in the Phoenix area as early as March I was surprise to see-much earlier than coastal soucal and FAR earlier than the June-July of Hayward ca.