Terriers should be Mentally and Physically mature before being entered to quarry.
In most cases this will not be before the dog reaches 14 months minimum.
Some may enter before, we've all heard of the macho man whoes dog entered at 7 or 8 months, the dogs if they ever did enter that old, will have a short working life as alot of early entered terriers will spew it after a season or two. being over matched by their owners . JMO