Ususally not. Thymus usually regresses after reaching maximum size at puberty. It becomes infiltrated with fat during regression. It can enlarge with tumors such as thymoma and thymolipoma. Also hormone associated with hyperthyroidism or graves disease in adolescence can stimulate thymus to overgrow. Otherwise thymus remains dormant in older people.
Usually not. The thymus regresses as we get older but there is a respectable population that has something known as a thymic remnant. It is more than likely that your thymus might never have regressed.
Not exactly. The thymus starts out fairly large in childhood. As one ages it slowly shrinks, until it is almost completely gone by adulthood. Even without a thymus, the adult immune system (of which the gland is a part) work normally. It does not normally grow back, but there are certain conditions (e.g., myasthenia gravis) where a tumor of the thymus (thymoma) may occur; these are usually treated surgically.