A strong advisory practice is built on mature relationships. However, if the average tenure indicates only long-term relationships and there are no new relationships being formed, the firm is at risk of losing future business (one might ponder "Why isn't the firm adding new clients?"). Conversely, it might be a red flag if the firm is well-established, but doesn't have long-standing client relationships (i.e., "Why aren't clients staying?").
As you continue to grow the firm and add new clients, you'll automatically diversify your client base with a blend of newer and older relationships. Bringing on your clients' next gen as clients themselves (e.g., securing an existing client's 20-30 year old children as clients), is often an easy way to start. For a business to really thrive, it needs to be constantly growing yet also strengthening it's existing relationships. And once you've established a relationship, be sure to nurture it so you can retain it.