The crux is that welfare payouts need to be wholly discontinued in order for UBI to make sense -- the intention is that UBI replaces a unmaintainable patchwork with a universal safety net. Having both doesn't make economic sense and only causes confusion and financial pain.
One of the main risks of a minimum wage is suppressing the employment rate of less-skilled, less-experienced workers. If the minimum wage is too high, it's uneconomic to take on staff who don't deliver enough value for the business. The reduced rates for young people and apprentices exist to reduce this stifling effect - without them, you'd make it uneconomic for businesses to take a chance on new employees who might need a lot of training or support.
This does of course create a secondary risk of businesses laying off older staff in favour of young people and "apprentices", but I haven't seen any clear evidence that this is a widespread problem.
If you've never seen any evidence of that, you've not looked very hard: This is what every supermarket does for their filler jobs, and to a lesser extent for their till and other store facing jobs as well. If you're over 18, you have to be very lucky to keep your job (or more realistically, to keep being given work) in such a situation.
Outside of supermarkets this sort of practice is also pretty entrenched in a lot of small-medium IT businesses: They have a sizeable portion of their workforce as trainees or whatever (they may be underpaid legally, so they almost always are) and don't hire them after their time is up (ever, as a rule). New trainees are then cycled into the slots that are now vacant.
It's to do with employment rates and avoiding financial damage to small/low margin businesses, there's a commission of some kind that tries to work out the maximum viable level.
But since its conception I only read good things about it.