I did it aged 16.
It was fun but really the useful bit was holding down two part time jobs (waitress and babysitting) for 16 months in the run up to it to pay for it. And not spending the money I earnt - it taught me to save!
The charity bit is total window dressing, we painted a school and played football with the kids. We were 16 - we had no actual useful skills!
The trekking was hard but rewarding and there was some resilience required in getting up, putting wet socks on and trudging onwards on morning where it rained hard.
But really the working and saving bit was the most useful life lesson.
I paid for it all myself (my parents paid for boots, rucksack, clothing etc).
Most people had parents pay a chunk of it and saved birth, Xmas money plus part time job money (easier if you are 16 and can get proper jobs in cafes/shops etc, hard when under 16. People worked in supermarkets, cafes, the local library, etc. Suspect it may be harder to get those jobs now though).
We did one fundraising quiz and made about 50 quid per student. It was done as a team building exercise really, having to plan and organise it. It would never had paid for any of us to go.
But it is a holiday. There was no noticeable difference is useful life skills or awareness of the world between us and the students who didn't go by the time we sat our a levels.
Doing DofE in the UK would achieve the same ends but cheaper.