Turns out there is no official age, but in an interview the author duo (it's a writer and an artist that just use a single combined name, if the bear has glasses its the writer only) they said they only started after they were over 20. So they are definitely in their 40s.
Apparently their motivation for becoming manga artists was, the writer heard you could make decent money as a manga artist, and the artist had zero motivation to do anything special but was up for making more money.