Two very different situations
The same technique serves two unrelated contexts. Medical preservation precedes treatment that will destroy fertility — chemotherapy, radiotherapy, gonadal surgery, gender transition. It is offered everywhere and generally funded. Elective preservation, with no medical reason, is permitted in some countries only, rarely funded, and its results depend entirely on the age at which it is done.
Sperm freezing
Simple, quick, inexpensive: one to three samples, an analysis, storage in liquid nitrogen. It should be offered systematically before any gonadotoxic treatment, including in adolescents. Where no sample is possible, surgical testicular retrieval sometimes finds sperm, usable later with ICSI.
Egg vitrification
The path is the first half of an IVF cycle: stimulation, monitoring, collection — without fertilisation. Mature eggs are vitrified individually.
Age is the only factor that really counts. Under 35 the eggs are good and relatively few are needed; at 38 markedly more are needed for the same chance of a child; after 40 the number required is often out of reach in a single cycle. Many women come too late, because the decision rarely presents itself at 32.
Two numbers should be given before you sign: how many eggs were stored, and the estimated probability of a live birth with that number at that age. An offer selling "insurance" without both figures is selling an illusion — a large share of women who freeze never come back for their eggs.
Ovarian tissue
A fragment of ovarian cortex is taken laparoscopically, cut and frozen; it can be grafted back after recovery and restore both periods and fertility. It is the only option before puberty, and the only one when treatment cannot wait the two weeks a stimulation takes. The technique has left the experimental stage, with hundreds of births reported worldwide. One caution remains: in leukaemia the tissue may carry malignant cells, and regrafting is then weighed case by case.
For how long, and then what?
Storage limits, annual consent renewals and what happens to gametes on death, separation or non-response are matters of national law. These are exactly the points couples do not ask about at the start, and that cause trouble ten years later.