A) Yes you can improve but you need to practice. The problem with singing is anyone can do it. It is not like a guitar where you cannot just pick it up one day and sort of play. You have to take lesson and practice. We all sing along with CD's or the radio. But if you want to get better, you need to treat your voice like any other instrument you'd like to play.
b) You can work to expand your range, however, your range will decrease with age. That is the reason why U2 have dropped the key thay play some songs in now. A song like Sunday Bloody Sunday is several 1/2 steps below the original key.
You can extrand your range by practicing scales. It all goes back to the 1st point of treating your voice like an other instrument. You have to practice to get the most out of it.
Best of luck to you