I would not hesitate to do the dental at 5 years old. You can get a fresh start and then brush every day going forward. Maizie's teeth are pretty good, but Frosty's are genetically less good and he may have to have a dental in the next couple of years (his brother had to have a dental I believe before he was 2!).
Dentals are very safe. I just had one done on Zooey, who is 12. The recovery was more hellish on an older dog, but the younger ones bounce right back, especially if they don't need any extractions or dental surgery. As with us, it's better to do regular maintenance on our teeth rather than wait until things get bad!