@SleepingStandingUp
Mackenzie - she had her gender surgery in 2020 when she became 18 years old, so would now be 21, but with the two-year jump will be 23. This fits with her now working as a lawyer apparently, as she would have already graduated from university in the programme's gap.
Sadie - when she arrived in 2022 she was still at school and would have been 15, so should now be 16, but two-year jump makes her 18. But we haven't seen her much at all yet so she could be at uni or working.
Holly - she was born in 2007 (when Karl and Susan were getting remarried in London) so she should actually only be 16! But she's been rapid-aged each time she reappears. When she guest starred as a child she was older, then again later when she should have been 15 she was apparently 19, some reference made in the past to her studying medicine at university like her father. So now she must be in her early 20s on screen, though presumably dropped out of medical school as she's working at the hotel.
Byron - I don't think any reference has been made to the ages of either Byron or Nicolette. But their mother, Jane, left Ramsay St childless and single in 1989, so both her children cannot be over 30 realistically.
Cafe Dude - "Haz", we haven't seen enough of him to know. But he's grouped with the characters Mackenzie and Sadie so far so I'd guess he will be early/mid 20s.