Is pediasure okay to use as a 19 year old trying to gain weight?

Pediasure : Sure but you can add weight with adding butter, olive oil, cheese and peanut butter to your meals

Not a good idea: won't help, as it is designed for failing to thrive infants and toddlers who are still developing, not for adults as yourself. Seeing your doctor for assessment, who may refer you to an endocrinologist / dietician is what you need, however no harm consuming pediasure, except for its cost, wish you well