As another mom living the life w/ a child w/ ADHD (and Aspergers, Anxiety, Executive Function Disorder and on and on), you've gotten great advice already.
Have your child fully tested by the best resources you can find (and afford) ASAP. Knowing the full picture makes a HUGE difference in determining how to help your child succeed. As Suzanne pointed out, ADHD often co-exists w/ other significant issues that affect learning, socialization, physical coordination, life skill development, emotional control, etc. Without the full picture, you're just shooting in the dark, hoping to solve all his school problems by focusing on that one issue. (Suppose a child also has dyslexia or an auditory learning disorder--ADHD treatment won't do anything to help those issues.)
Frankly, ADHD is the least of my DS's problems. I was initially 100% opposed to medication for him, but I came to understand that without it, he couldn't do what was required of him (sit still, follow directions, stay focused, work neatly, not bother others, listen quietly, pay attention). It would be like telling a nearsighted child to "try harder to see the board" or telling someone w/ a broken leg to "try harder to run". It's not a matter of lack of effort or laziness--it's a physical difference in how their brain is wired!!
W/ medication, DS is an A student who will be taking honors classes in Algebra and Science next year (in 8th grade). He helps other students w/ their homework and he won't do anything else after school until his own homework is done. Socially, he still has a ton of issues, but at least he can look at his academic success and be proud of what's he's done--on his own.
We still monitor his medicine (dosage, type, duration), and make changes from time to time based on what we see and find out about from the school. So, if one medication works for awhile, you may find over time that side effects develop or the dosage needs tweaking (up or down) so you do need to be vigilant.
ITA that kids who need help for learning disorders but don't get any are most often the ones who develop significant problems in adolescence. They turn to drugs, alcohol, sex, criminal behavior to self medicate because they aren't able to succeed in school, are misunderstood at home, have trouble keeping friends, you name it.
Some teachers are better than others in understanding how learning issues affect classroom behavior & performance. I've had some utter gems and some absolute horror shows. Don't be afraid to push for what your son needs and if he's not getting it, speak up as often and loudly as you have to.
Also, LDs (learning disorders) are most often discovered whenever there's a big leap in expectations for independent work from children (eg. 3rd grade, middle school). That's why there are some "magic ages" (your phrase) for these diagnoses.
Finally, I know you are seriously stressing this diagnosis. Please, take a few hours and look online for ADHD support forums and information. There is a wealth of information out there by knowledgeable people who have walked in your shoes. Sadly, there is also a lot of misinformation by others who like to take advantage of parents and promise quick cures & easy natural solutions. Having tried more than a few, I can tell you to save your $$ for actual doctors, psychiatrists, neurologists, psychologists, etc.
Good luck and (((((((hugs))))))))).
