I don't know if you can surmise that older people can't become grandmasters. I doubt that older people have the time that younger people do to practice. Also, as the article talked about, older people are less likely to be vulnerable (ie temporarily look dumb) in order to get better.

So I think, there isn't enough data to conclude that someone older can't be a grandmaster. Though, most people won't be a grandmaster either way, so there aren't that many data points to go off of.