--min-age
only works on files not on directories so this approach won't work
Also purge does not obey the filters anyway.
You can use delete + rmdirs but this may not do exactly what you want...
What you really want is to write a script which enumerates the backups you want to keep then excludes those and deletes the rest.
Here are some things which might help
# get a list of the 30 most recent dates
for i in `seq 0 30`; do date +%Y-%m-%d-\* -d "now +$i days" ; done | sort > dirs-to-keep
# list all directories
rclone lsf --dirs-only --dir-slash=false remote ${dest}/old | sort > all-dirs