This is a band-aid approach; the long time-to-upgrade times made sense long ago when there were fewer Ages (when, say, the max age was Industrial...), but it makes zero sense today. Folks should have a solid Industrial base in 2-3 weeks today to increase the user base and fun. Always keep longer times in the last few ages but make the earlier ages quicker. The choice is simple: have folks reach Industrial Age and enjoy the game (and yes, even start spending more), or have them quit early at say Classical Age. It makes sense for both users and business.