Phase 1: Test batting till 1910s was underdeveloped and cricket was too much in favor of batsmen. So, 1870s to 1910s have lowest averages, but batting was consistently improving.
Phase 2: Batting improved a lot in Bradman era (1920s-1940s) and this was batsmen dominated phase.
Phase 3: 1950s. Pace bowling started developing from trundlers to faster bowlers and batsmen struggled to manage them. Averages dropped quite significantly with improved quality of bowling on uncovered pitches.
Phase 4: 1960s. Covered pitches came into picture and bowlers had no clue to what to do. Batsmen made merry and it was much better decade of batsmen than previous one.
Phase 5: 1970s-1980s. Balanced time for batsmen and bowlers. Batting averages went up and pacers found their way to get wickets.
Phase 6: 1990s. Toughest era for batting with all teams having good bowlers and batsmen struggling. Batting averages dropped significantly. Averaging 50 became almost impossible.
Phase 7: 2003-2013. Era of FTBs. Many great bowlers retired. Pitch quality improved a lot and we got flat pitches all over the world. Minnows entered the scene with lot of matches. Top 6 team averaged 37 with bat. Great ones averaged above 55 and there were million batsmen averaging 50+. Steyn was only great fast bowler emerging during this phase. Massive FTBs and minnow bashing era.
Phase 8: Post 2013. Teams becoming desperate for home advantage and started preparing spicy pitches everywhere. Bangladesh improved at home and Zimbabwe rarely got matches, so that minnow factor dimnished. So many new pacers and spinners emerging again. Overall batting average since 2013 - 33.10. The way it's decreasing with year, it is easily toughest phase in 1990s.