It would be very useful, if age supported reading password from file descriptor/path.
It can be done safely, it would be useful for automation and AFAIU it is the only encryption mode that actually does authenticated encryption.
AFAIK types of usage shown below should be safe. Both can less or more practically be read by the same user, but a more permanent private key would be even easier to read.
age --password-fd <( <<< "$PASS") ...
pwfile=$(mktemp /dev/shm/XXXXXX)
cat > "$pwfile" << EOF
$PASS
EOF
age --password-fd "$pwfile" ...
rm "$pwfile"