Earlier than feels necessary. Assembling the investor's evidence, completing the transaction, obtaining a decision and travelling all take unpredictable time, and no stage carries a promised date. Then the child's own matter needs its own runway on top. Counting backwards from the birthday through both processes usually shows the work should already have begun.
Build the schedule backwards from the birthday
Put the birthday at the end of the calendar and work back through the child's application, its preparation, arrival, the parent's decision, submission and evidence gathering. Where the sequence no longer fits, decide what changes: an earlier filing, a different order of steps, or accepting that the child may follow a separate path. Review the schedule whenever a fact shifts, since a delayed closing on the business moves every date behind it.
There is a further reason to start early that is easy to miss. Where the child's own route requires a period of study, an offer of admission or a qualifying period of some kind, that requirement runs on an academic or professional calendar with fixed intakes, not on a schedule that can be adjusted to fit the parents' transaction. Missing an intake can cost a year, which is more than most business delays cost.
Identify any such fixed date at the same time as the birthday, and treat the earlier of the two as the one the whole plan is built around.