Why do I seem to be able to hold less air to finish lines of a song than I used to?
It may be that you are creating tension in your lungs which causes more air to escape through pressure on your vocal cords. There are lots of breathing techniques you can practice to improve this including, but not limited to:
Breathe in on 5 beats exhale on 20 beats. Blow up a balloon using gentle steady breaths. Swimming! - not the kind where you keep your head out of the water but real head under the water at each alternate stroke (front crawl and breast stroke are best). If you swim regularly this will increase lung capacity, breath control and stamina. Yoga is another exercise which helps you to control breathing.
You can also check out the site below for general advice on singing and breathing improvement. I know that the same thing happened to me as I got older, and it was probably a result of bad breathing habits which didn't effect me earlier in life when I was in better physical shape. These exercises help me to continue church choir and the occasional karoake scene...I feel confident that they will help you, too. Good Luck!