The form "iuvenis" is both male and female so it doesn't make sense to offer the options "solus" and "sola" as both are correct.
[ Iuvenis sola non habitat ] • Duo rejects using sola here.
[ Iuvenis ] refers to a youth of either gender as far as I my searches reveal
In some Medieval Latin society etiquette greatly restrained comment or discussion of the highly protected daughters of the elite. Doing so brought dishonour.
Mention of marriageable age [ post puberty ] female youths is sparse.
In a 1299 inquest of a matter of subject named Strella, who was suspected of indiscretions; a witness in response to a question about her age and appearance termed Strella:,
[ una iuvenis femina ], and very beautiful.
The Premodern Teenager, 1150-1650, google book