The age of this one is known, although it’s not the weeping one. It was planted in 1760 in Kew Gardens.

I recently walked around the ramparts of Lucca, Italy, which are planted with the species. Their shade was very welcome in the 39c heat.

Yes, Styphnolobium japonicum flowers around this time (at least where I live).

On the website 'Monumental Trees' you can see more pictures of old Styphnolobium japonicum 'Pendulum' in Europe.

MonumentalTrees / Styphnolobium jap. Pendulum

It has been a hot time in Europe, with high temps consistently between 36-38C. I’m ready for some cool midwest weather.

I'd guess the top one is more than just several decades old. Prb'ly alot of decades.

This tree could be well over 100 years old. The park, which was designed in 1830, is the oldest in Pamplona.

'I wonder if the seeds from 'pendulum' would produce weeping seedlings?'

According to this source, not likely.

'Varieties

The weeping form of the Pagoda tree was particularly suited to the picturesque tastes of the Victorian era, and was more commonly used than the species during that period. Styphnolobium japonicum 'Pendula' had first been cultivated in China; the plant-hunger Robert Fortune reported seeing a weeping Pagoda tree in Shanghai during his visit to that city in 1853. (53) Although it seldom flowers, (54) and is difficult to propagate and expensive, Scott called this variety:

the finest of small pendulous trees. ... [Its] branches are green and somewhat angular or crooked so that in winter the tree has a somewhat knotted and curious look. ... It is at the same time symmetrical and picturesque. (55)

Sargent praised it as well:

It has long pendulous shoot ... We hardly know anything more ornamental or striking; even in winter, the long slender branches of beautiful bright green render it most attractive. (56)

'Pendula' is still used today as a formal specimen or living sculpture. Its convoluted branching structure makes very little growth on its own, so it must be grafted to seedling understock at the desired ultimate height of the resulting tree.(57)'

Not a secure site: http://www.leslie-turek.com/LandscapePapers/PagodaTree.html