public Duration age(Instant now) {
        Instant oldestEventTime = events.values().stream().map(Event::at).sorted().findFirst().orElse(now);
        return Duration.between(oldestEventTime, now);
    }
Returns the age of this node as best as we can determine: The time since the first event registered for it
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/node/History.java