Would either marfans or hypothyroidism & its meds keep an adult's growth plates from closing normally thus resulting in added growth well into 30s/40s?

Probably not: With diagnoses like Marfans and hypothyroid, you need an excellent doctor. If you are truly gaining height at this age it is VERY unusual. One condition causing this is pituitary secretion of growth hormone, but rarely growth hormone may be produced elsewhere. See a specialist in Endocrine disorders, possibly at a University/ Med School ASAP. Be well!