When there is a potential epidemy, like it is the case actually ( the confirmed polio cases where detected in syria and not in lebanon, but the world health organization WHO imposed increased protection in the whole region to prevent dissemination), targeting children below the age of 5 will help:
1. Protect those not fully immunized yet
2. Provide an indirect protection to others ( members of the same household) because oral polio will be excreted in the feces for 6 weeks and boost the immunity of members of the same family
3. Prevent the dissemination of the disease knowing that kids are the ones who get contaminated first because they are the most exposed ( close contact in schools and nursery)
So to answer your question: no need for adults to be vaccinated but again, there is no harm in getting a booster dose, and you can always discuss this with your doctor.