Our youngest child is 18 months old and came to us already treated, so children can show symptoms early.
Anxiety disorders, depression can cause the same appearance of the same symptoms (as ADHD), but you have a whole different reason for these, so treatment is dramatically different.
The frontal lobe of the brain, which controls these behaviors, matures until age 21. As children mature, some symptoms get better -- though they don't grow out of ADHD, as used to be believed, and half need treatment into adulthood.