In post-apartheid South Africa 'youth' was classified as up to 35, to recognise the lack of education and opportunities for many communities under apartheid - which meant that a lot of people might be still completing their secondary education and tertiary education/training a lot longer than would have been the case in a more equal society. So it was chosen quite specifically as a more generous definition of 'youth' than would usually be considered.
A quick Google with a fairly random set of organisations coming up:
Young Farmers Clubs have a cut off of 28 😂and they extend their definition of 'young' further than most youth clubs.
UK Youth programmes - doesn't give much info about age ranges, but Young Changemakers is aged 16-25
Buttle Trust grants up to the age of 20
This list of Trusts that funds work with 'young people' lists 30 funders, nine of them specify a maximum age, none of them categorise 'young people' as any older than 25.