A virus that cuts off Telomeres that are too short.
To quote Wikipedia: "A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. [...] For vertebrates, the sequence of nucleotides in telomeres is TTAGGG. This sequence of TTAGGG is repeated approximately 2,500 times in humans."
Very roughly speaking: when cells replicate, the telomeres are shortened. This means that in young organisms, telomeres are long, and in older ones they are shorter.
So... assume a virus - like a common flu virus - that has been mutated and targets cells with too short telomeres and simply cuts them off. Cell replication will be shot to hell and your chromosomes in each cell afflicted by the virus will be a jumble; the cells die. Once it hits it will probably be like a severe hemorrhagic fever (like Ebola or Lassa). If your telomers are long enough, you are not afflicted at all, your body adopts normal immunity to the virus and defeats it.
Weaknesses in this: individual variation, individual immunity. There cannot be a hard limit that says "Until 18 years, 0 months, 0 days you'll be fine... above that - one day later - it is 100% mortality".
But allowing some fuzziness in the outcome - such as a few kids die and a few adults survive - this could perhaps be sufficiently credible. To increase the credibility and avoid nosey questions - especially since cells replicate at wildly different rates in the body - you can have the virus target a specific organ, like the brain (compare to the movie Contagion) or the heart.