@Override
  public String toString() {
    AgeSnapshot snapshot = getAgeAtEvictionSnapshot();
    return "hitCount=" + getHitCount() + ", hitCachingCount=" + getHitCachingCount()
      + ", missCount=" + getMissCount() + ", missCachingCount=" + getMissCachingCount()
      + ", evictionCount=" + getEvictionCount() + ", evictedBlockCount=" + getEvictedCount()
      + ", primaryMissCount=" + getPrimaryMissCount() + ", primaryHitCount=" + getPrimaryHitCount()
      + ", evictedAgeMean=" + snapshot.getMean();
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java