I'm slightly farsighted, will my vision get worse?

Depends: People generally seem to become more farsighted as they age, and the lenses of their eyes get more stiff. However, as cataracts develop, the lens of the eye tends to become more nearsighted. These two processes happen at the same time. Whichever process happens to dominate will determine whether you become near or far sighted with age, and either process may dominate at different stages of life.

Presbyopia with age: Presbyopia is distinct from farsightedness and occurs usually in patients in 40 Y.O. Or older when lens behind pupil thickens and results in impaired abilty to focus at near objects and small font reading tasks. Presbyopia onset variable depending on pre-existing level of myopia (nearsightedness) or hyperopia (farsightedness) and can be addressed by progressive multifocal lens glasses or lasik.