Actually mastrubation does not causes the problems you are talking about, As far as I can say this might be somewhat Psychological problem. Your wife as you said not like sex anymore maybe because of her age and psychology. The best advice I can provide to you is make your and your wife's relation much emotional first and then try to get physical with her, she might have vaginal dryness
or little or no orgasm but you can take her hand for your sexual pleasure, last but not least is Emotional attachment is first & most important . There is no age to enjoy sexual pleasure for a Man or Woman.