Due to the fact that these are play-groups, it is not recommended that a dog over the age of 10 join the groups. Most of the dogs in my groups are between puppyhood and age three, which means high-energy play, and lots of on-the-ground wrestling. Older dogs tend to show discomfort with this, and can even act out aggressively to get them to stop playing.
Additionally, dogs can show signs of cognitive decline with age just as humans can, which can often lead to behavior changes that include confusion, anxiety, hallucinations, not recognizing people, and aggression. That level of unpredictability is not safe for the group.