How well a stimulant works doesn’t really have much to do with age or weight – it’s more about how your individual brain chemistry responds to the meds. A skinny little kid might need the max dose to see improvement in symptoms while a large middle-aged man could be just fine on the minimum dose.
When I started out, I started with with the smallest dose of Concerta and worked up until I got to the point where my symptoms were being managed and I wasn’t experiencing side effects. The small doses would be great for a few days or weeks, but then I’d not get the same benefits from them, and that’s when you know you need to up the dose. Then I went too high, had a horrible month of insomnia and extreme irritability, and went back down again. I got Ritalin as an as-needed afternoon booster for the Concerta, which was helpful, and much more tolerable than taking the higher dose of Concerta.
That said, have you looked at Concerta? At the right dose, I find it more effective and smoother than the IR Ritalin alone, and I’ve been on the same dose for about five years now. Caveat is that you have to get brand name or the good generic (that uses the right release mechanism in the capsule) because the rest of the generics are complete garbage.