I just started cycling last September and still use my HR monitor every ride. I'm 54, my max heart rate is 183 and my resting heart rate is 67. I haven't checked either since shortly after I started riding, so it's probably time to check them again.
I do 3 weekday evening rides of 17 miles. Two are at recovery pace HR's of 130's to 140's. On the other ride, I keep my HR above 160. I do one longer weekend ride of 40 to 60 miles (a club ride), and mix in some hard sprints with easy riding.
I think a heart rate monitor really helps, if you plan on training very seriously at all. I've noticed that sometimes, I feel great while holding that above 160 pace, and other times I feel like I'm really putting out a hard effort, but the HR monitor only shows 150. That tells me that my perceived effort is not a very reliable indicator of how hard I'm riding; the HR monitor is much better. I'm really not riding enough hours for overtraining to become a factor, but I still like keeping up with the easy data the HR monitor gives me.
It's funny how quickly my casual riding for fitness turned into really trying to become a strong rider. It's a testament to the fun of bike riding, I think.