Narrowing of the vagina with age is usually due to the drop in estrogen with menopause and is not usually a problem at your age. Masturbation won’t prevent narrowing. Having intercourse or trying a dilator helps if it’s a problem. Estrogen therapy at menopause or ThermiVa procedure can help too.

38 is still very young. The vagina naturally elongates with arousal and they do sell vaginal dilators which you can use.

That's usually after menopause (average age for menopause is 51) and even then only becomes a problem in some women. Sometimes hormone replacement can help prevent that.

The vagina should not shorten due to age. There may be other reasons and suggest seeing a gynecologist to evaluate you. This is not direct medical advice.

Unfortunately masturbation will not help with this. You would need a set of vaginal dilators