In my opinion, sub-20 at age 40 is solid. However, how you rate a sub-20 5K at age 40, all depends on who you choose to compare yourself to.
Compared to the general population, sub-20 at 40 is excellent. 99.9% of men or women at age 40, wouldn't be able to run sub-20. Most are overweight or obese, cardiovascular invalids and don't exercise at all. They'd be lucky to break 30 min. But they'll beat you at beer pong, 19 out of 20 times.
For 40 year old men who actually run, it's still pretty good. Most guys at 40 that show up to a local 5K aren't going to break 20. You might even get lucky at win Masters on occasion, at a small local 5K. But most aren't training hard, either. Even at a bigger 5K, you're still going to be above average in the masters division, and in your age group, at 19:59.
However, when compared to elite open, or even elite Masters runners, no, 19:59 is not good at 40.
Choose who you want to compare yourself to.