Thanks for your input, Timo.
I have grafted Violeta on a sucker of an adult tree. It's only a sucker, but the tree has more than 30 years, so it can benefit from part of the good root system and has lots more energy to grow than usual.
I don't think it's comparable with grafting on a cutting, even a good variety that roots and grows well.
Grafting directly on an adult tree has given me even better results in terms of speed of production (i had cuttings producing figs 5 months after being grafted) and overall aspect of the cultivars grafted, maybe this extra energy can explain, in part, why the graft of Violeta thinks its an older tree ;.)
An example...
This cutting of Negronne was grafted in March, had is first leafs in April and has figs in July: