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According the the Tier 5 (Youth Mobility Scheme) page:
How long you can stay
Youβll be given a visa to live and work in the UK for up to 24 months.
You can enter the UK at any time while your visa is valid, and leave and come back at any time during your stay.
If you turn 31 while youβre in the UK, you can stay for as long as your visa is valid.