Before the birthday, and with advice taken well ahead of it. The programme's terms have been stated and restated since its introduction, so a decision made on last month's understanding may be wrong, and the family's own deadline is fixed and unmovable.
One deadline is legal, the other is the programme's own instability
Two clocks run at different speeds. The family clock is precise: a child ceases to be an eligible accompanying child at twenty-one, and no amount of goodwill moves that date. Working backwards from it, the legal assessment, the documentary preparation and the vetting all need to fit inside the remaining months, which is tighter than it sounds.
The programme clock is the opposite of precise. This is a recent framework whose published terms have changed, so anything Ilse read a few months ago should be checked again against the official page and FAQ rather than relied on. The sensible order is unchanged by either clock: obtain a legal assessment of the underlying classification, confirm the current official terms, and only then pay a fee that will not be returned.