Age is not related. But yes, it does. Coffee is a stimulant to most and a relaxant to those with ADD/ADHD, due to chemicals in the brain working differently to the average person.
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Additional:
I tend rarely to diagnose children with psychiatric disorders because the whole thought of that is an additional worry; along side ADHD (Attention deficit hyperactivity disorder)... Most people are diagnosed at a time when Hyperactivity is very common (IE <9-ish) and have the label follow into adulthood. I'm not saying there is not such thing, but it's a controversial subject...
Back to the question, yes the main medicine treatment for ADHD includes stimulant drugs... But non-stimulants like atomoxetine are used as alternatives.