Age grading is an interesting topic, missed the discussion yesterday because I was at work *trying* to stay focused. As far as 5K, at 40-42 I still kind of liked 5Ks and I focused most of my training year on 5K and 10K. By 50 they indeed got more difficult and other than once or twice a year, when I'd actually go for a time, became more of a training race. I've sort dreaded them the past years, but after having some decent results thinking about a little more emphasis on the track and roads.
As far as comparative purposes, at a little older now, the 5K and marathon are not too far off (at least in the same wheel house). The recent 5K on course that has 3 hills over the last mile on a warm morning (70s in Atlanta) equates to 2:47 for a marathon. Age grading I'd need to run a 2:45:00 or faster, likely not happening especially in Atlanta. But on a Daniels chart it's a 2:47:30. On a fast course like Chicago or one of the big European races that might be possible.
Marathon indeed does have more draw/prestige than a road or track 5K as a masters (unless the athlete is setting records). A world or USA age group record at a shorter gets more attention and wow than being close but not quite at the record for a marathon.
And outliers do skew the marks somewhat. Martin Reese went on a tear earlier this decade with a series of 1:11 to 1:13 half marathons at 60+ (I think he holds 6 of the top 7 times in the world), so age grading in that distance is really tough.