Anquetil tells Nicola that Mariners belongs to Sir Charles, and that Lewis is his youngest son. Like you I think this may be quite a lot younger so perhaps a "functional only". Anquetil and Foley sailed together, so that Robert was the only non-Foley to know about the secret way in, so yes, I think Lewis was at Mariners as a child.

There seems to be no mother for either Robert or Lewis though: which might have drawn them together too? There's a whole thread to be written on Mothers in AF's universe one day (if not already done!)