The maintenance obligation generally ends at adulthood. However, many children have not completed initial education then, so the obligation continues.
After adulthood, support is called adult-child maintenance and follows somewhat different rules. It ranks behind former-spouse maintenance if money is insufficient for both. It is also capped: an adult child cannot share without limit in the payer's surplus income.
Unlike maintenance for minors, generally always owed, adult-child maintenance has other requirements.
It is owed only if parental finances permit and the personal relationship makes support reasonable. An adult child who unjustifiably broke off contact may therefore receive no maintenance.
The child must also be suited to the initial education and seriously pursue completion.
In short: Maintenance is generally always due until adulthood. If the child has not completed initial education on reaching adulthood but pursues it seriously, and support is reasonable financially and in terms of the personal relationship, maintenance continues. In tighter finances, it then ranks behind post-marital maintenance.