public void evicted(final long t, boolean primary) {
    if (t > this.startTime) {
      this.ageAtEviction.add((t - this.startTime) / BlockCacheUtil.NANOS_PER_SECOND, 1);
    }
    this.evictedBlockCount.increment();
    if (primary) {
      primaryEvictedBlockCount.increment();
    }
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java