I feel like there is a very simple answer to this question. When people come to my shop and ask me if they need a new tip, I generally tell them that a tip is still "good" as long as they still like the way it plays. I have replaced hundreds of tips over the years, and people come to me all the time asking me for advice on the condition of their tip. Generally if they don't like something about their current tip, I will try to correct whatever issue they are having such as mushrooming, flattening of the dome, etc.
Once all of the obvious visible corrections are made and the tip is brought back to their preferred shape, if they still do not like the feel of the tip then I would suggest changing it.
Another thing to keep in mind is that all tips change over time. The feel of a tip when it is new is usually noticeably different than how it plays a few months down the line. Most tips generally become slightly harder as they are compressed by thousands of shots.
I would strongly argue that a tip does not necessarily "need" to be changed just because it is old or getting thin. The tip is quite possibly the most personally-important part of a cue. Every player has different preferences. Regardless of what anyone on this board may tell you, there are no arbitrary rules or standards about when a tip expires or goes bad. I have shot with cues that had 30 year old tips, and I loved the hit. I have installed brand new tips that played like doo doo.
To sum it up, just trust your gut feeling. Something about your tip must be bugging you, or you wouldn't have posted. My advice is to find a cuesmith who can work with you to try to discover if there is a basic maintenance issue that can easily be corrected with your current tip. If the basic stuff doesn't improve your impression of the tip, then you should change it to something you like better. Hitting a ball should leave you with a positive feeling about your tip. Any circumstance that prevents you from feeling positive about your tip will impact your game.
Just don't be pressured into changing tips solely because it is getting old. Some things actually improve with age!