Have a 14 year old DD with ADHD.
To be honest there was no real question over whether she was or not - it was incredibly obvious since the age of about 5, and I was tearing my hair out and exhausted by the time we got the Dx done.
She's better at school than out of school, so they weren't pushing for us to see anyone, but also didn't say we were being ridiculous - and now she's medicated they can all suddenly see a huge difference.
I think the things to bear in mind are:
Will you medicate? Probably not much point in pursuing a Dx if you don't want to go down that route as you can implement all the strategies yourself with a book otherwise (only thing that worked for us was huge amounts of tech for alarms, calendars etc and me taking over all life admin and regarding DD as being at least 2 years younger than she is).
The waiting lists are horrendous - 2-3 years on NHS in my area, and if you don't score high enough on the assessment questionnaires from both parents and school, you don't get into the system at all.
Private is quicker - about a 3 month wait, but expensive. We have been paying out over £100 a month for DD's meds. If I add up assessment, follow ups, meds etc, it's north of £2k for the first year, and around £600 a year once we move to shared care.
For us, we chose to go for the Dx in the end due to DD's achievements (or lack of) at school being totally out of synch with her ability and her rising anxiety levels.