Don't take this the wrong way, but, have you done any research into the industry? I mean, you haven't even mentioned any specific type of modeling, which sets off muchos alarm bells in my head.
If those are yr stats, you'd probably have to look for print jobs, primarily. Maybe some catalog work.
See, if you are "built", then you'll be too big for runway or editorial.
And, fitness... eeeehhhhh. At 6'4" you probably don't have ideal proportions. I work with fitness models, and most of the boys are between 5'6" and 5'10". Those are just the heights where everything tends to be in near-ideal proportion.
You could MAYBE break into fitness if you have some name recognition from, say, a professional sport.
Otherwise, print would be the way to go. That's a crowded and competitive field. You'll go nowhere unless hustle=lyf for the first year or two.
And even then, you might just be too big. Catalog models are usually 6'-6'2" and wear jacket size 40reg or 40long (euro 50).
Finally, yr question itself just cements my conclusion that you haven't done yr homework. What? 23? Too old? Whatever yr smoking, it must be good...
23 is about peak age for runway, and near peak for editorial, but yr too big for those anyway. For everything else, 23 is still pretty young. Yr not even all the way filled out yet, ffs.