I suppose there are people that do not feel comfortable in groups with a wide range of ages of participants. There are also groups for specific age ranges, such as 20-30, teens, seniors, etc.
So if a group is setup for a specific age group then it makes sense for its members or participants to be in that age range.
For groups that are NOT setup for specific age ranges, that is, open participation, then for someone to give you the boot or an unwelcome status based on your age seems on the face of it age discriminatory.
I suggest seeking out more welcoming groups, although sometimes a specific group may give you what you need if only its door was open to you. My sympathies to you.
Chrissy