My 9 year old son has expressed an interest in coming mountain biking with me. He currently has a bmx. I was thinking of building/buying something for him. What would people recommend for this age group? He’s tall for his age and approx 140cm tall.

Isla bikes or Frog is the usual answer.

What size wheels?
My 5 year old is on a 20″ isla and I haven’t looked into it but I guess her next bike will be a 24″. In this case I wont buy another isla and will maybe look at building something more future proof as she is growing so fast. So I will probably buy a frame and choose parts I can move onto another frame as she grows.
But then again with the high resale value of islas they are hard to look past.
Just looking on islas site they recommend there 26″ bike as age 9 onwards so maybe a small 26″ bike would be ok. if so you should be able to pick up a 2nd hand bargain with everyone being forced to believe the 650b hype.

IslaBike Beinn 26 was bought second hand here, and has done all three of our kids, and is now zipping round under another forumites 9 year old…
The geometry is great, they are pretty light.

You can also build up small bikes of similar :

Same lad, same place on Specialized 13″ womens frame:

The wee Trek that is owned by another forumite now:

Mrs_oab is only 148cm tall and rides a sweet old Cannondale F900SL

All the above were less than £200-250 to build up. An I sold most for £50-70 less than paid for, after three or more years use…

I am selling an Islabike Beinn 26 Small – email in profile if interested

EDIT – juts reread your post – for his height he’d be better on a Beinn 26 Large as I think he’d be out of seatpost on the Small by the time he was a little over 5ft tall

My 9 year old is on an extra small 26″ Cannondale now, and he’s tiny for his age, pretty much like Mrs OAB’s above (but in the classic blue) – that highly sloping top tube works really well. Again built up 1×9 from the bits box in the main, total cost well under £100 including frame&fork from an ad on here, 1×9 with new NW chainring, XT V on the front, Deore disk on the back.
Only the chain ring, grips and a youth saddle were bought new.

Cheers for the advice. I will take a look. If anyone sees anything for sale then please let me know.

The Genesis offering looks very smart to me. Still no massive need for a heavy underperforming fork but has proper brakes – http://www.genesisbikes.co.uk/bikes/youth/mountain/core-26.

Ordered my youngest a Frog bike for Xmas. It’s bloody impressive built it up this am before I left work as it’s stashed there. It’s every bit as good as her sister’s Isla.

I’m just finishing tarting up a specialized hotrock 20″ for my 6 year old, the stock parts are mostly heavy and a bit bobbins, but the frame is pretty good and it will take proper parts (from the spares bin/ebay/classifieds) it cost £50, plus maybe another £40 on new bits, couldn’t swing an Isla for that…

I am already eyeing up old hotrock 24s on eBay/Gumtree for a couple of years from now when she will need the next bike…

Otherwise I’d maybe look at a nice small aluminium 26″ frame that you can kit out with 24″ wheels and shorter cranks, once they hit the next growth spurt; bigger wheels and longer cranks to accommodate, so two bikes in one… ish.

Another shout for Frog bikes, my 7yr old has just outgrown his 20″ wheeled one and has asked for another as his next bike.