In Bangkok i have bought a grafted avocadotree from 40 cm tall with flowers. I picked the flowers off though but later in the same shop i saw some had fruit at the same length.

So if you graft these scions you might get the same, a just grafted plant with flowers. If you sure don't want that to happen then graft another time of the year.

But you can just pluck the flowers off and it will grow.

Yes, the fact it has not fruited yet does not matter.

Thank you. I will wait until fall.

If the seedling has more then 1 branch then you can graft onto one. Maybe it fails but then you can do the other branch(es) in autumn.

If it succeeds then you cut the ungrafted branches off or graft another variety onto those.

As long as you keep the tree's alive you can do many attempts.

Taking a bud off of a grafted tree that has yet to fruit will cause the bud to be more juvenile than the source of scion. But not by a whole lot.

The process is called rejuvenation and used on very hard to root plants. Scientists do the process many times over until the desired wood is young enough to root.

http://www.hort.cornell.edu/grafting/reasons/ReasonsGBLeft.html

2. Grafting for scion rejuvenation to facilitate subsequent rooting of cuttings ( Serial Grafting)

a. Ease of rooting is a general property of juvenile compared to adult growth.

b. Even though a scion from a mature tree tends to retain its adult growth phase.

As pointed out in the previous section, an adult scion will be slightlyrejuvenated by grafting onto a juvenile (seedling) understock. Because this rejuvenating effect is only slight a scion may have to be sequentially regrafted onto a series of juvenile rootstocks before a useful degree of rejuvenation has been achieved. Cuttings taken from this rejuvenated growth tend to root more easily than from mature growth.

c. This rootstock-influenced gradual rejuvenation of the scion is called serial grafting.

d. Serial grafting is an extreme (last resort) method for facilitating the rooting of cuttings from the mature wood of extremely difficult-to-root species such as 100 year-old Sequoia ( Tranvan, et al., 1991).