Can a person with lymphatic melformation not know that they have it until later on in life when the start developeing systoms when they are adult?

Not likely,,: Usually these are multiloculated cystic type of malformations that are seen before birth, infancy, or childhood. They often manifest as swelling and may or may not have other issues like breathing impairment, especially of on the neck. They may be treated with surgery, drainage of fluid, or just observation. By adulthood, these should have resolved, etc. And a lymph issue is from another cause.