I presume you are talking about someone who is raised Catholic but never underwent a regular First Holy Communion class, but at some point presented himself for Communion (which becomes, for him, his First Holy Communion, though maybe he is the only one who is aware of that fact).
This is irregular, but it is neither invalid nor illicit, provided the recipient has attended First Confession prior to receiving [per CCC 1457].
Canon Law (#914) requires Pastors (and parents) to make instruction available to children, but imposes no obligation for children to attend classes.
This is the only circumstance I can imagine where a Catholic in good standing who is old enough to receive Communion has not already made his First Holy Communion in the customary manner.