The default is always for players to play in their own year group with their peers at the same stage of rugby development. There are exceptions when they can combine as a group with other ages or an individual can play up or play down an age. These are limited and very specific with defined circumstances, assessments and approvals required before they are permitted.
Playing out of Age Grade is covered in Regulation 15.2 to 15.7. Please view Regulation 15 and Playing Out of Age Grade form