I guess I just don't see the point in the mist --- unless you decide you're likely to get the flu so you want to have it at a time that's more convenient --- get it over with --- but you might not get it at all otherwise. I haven't had the flu in years --- no flu shots either so why would I deliberately expose myself to the virus?
Or maybe the mist contains a virus that is considered less pathological, less aggressive so you deliberately get sick from that one to get protections against a more aggressive strain of the same type. Still --- if they advise the young and old not to take the mist --- they could catch it from someone else who did take it.
It's like the oral polio vaccine given to newborns --- everyone in the house gets a booster --- revaccinated -- when the baby is vaccinated because that's a live virus and isn't supposed to be given to babies living with someone who lacks an immune system.