Use current world_age instead of max

[vchuravy]

fixes #188

[codecov-commenter]

Codecov Report

Merging #189 (98f1e52) into master (52aaf98) will decrease coverage by 0.26%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   85.18%   84.91%   -0.27%     
==========================================
  Files           6        6              
  Lines         749      749              
==========================================
- Hits          638      636       -2     
- Misses        111      113       +2     

Impacted Files	Coverage Δ
src/overdub.jl	95.98% <66.66%> (-0.33%)	⬇️
src/tagging.jl	72.00% <0.00%> (-0.40%)	⬇️

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[vchuravy]

Might also fix: CliMA/Oceananigans.jl#1514 (comment)