code: @VisibleForTesting
      static Iterable<SSTableReader> filterOldSSTables(List<SSTableReader> sstables, long maxSSTableAge, long now)
      {
          if (maxSSTableAge == 0)
              return sstables;
          final long cutoff = now - maxSSTableAge;
          return Iterables.filter(sstables, new Predicate<SSTableReader>()
          {
              @Override
              public boolean apply(SSTableReader sstable)
              {
                  return sstable.getMaxTimestamp() >= cutoff;
              }
          });
      }
docs: Removes all sstables with max timestamp older than maxSSTableAge.
  @param sstables all sstables to consider
  @param maxSSTableAge the age in milliseconds when an SSTable stops participating in compactions
  @param now current time. SSTables with max timestamp less than (now - maxSSTableAge) are filtered.
  @return a list of sstables with the oldest sstables excluded