When a person is asexual it usually means that the person never was interested in sex with anyone.
However you could have a change of feelings and become asexual as in being disinterested in sexual activity or sex with anyone else after you've had sex before.
A friend of mine used to be sexually active and then later she said that she lost interest in sexual activity or any sex and said she's now asexual.
So it's possible I would say to become asexual after you've been sexually active before because some people may lose interest in sex after awhile.
Then again some people are never sexually active because they're all the time asexual from the beginning.
I have a brother who is not interested in sex and said he never would have sex so he would be asexual.