My 10-year-old needs a new mountain bike. What is my strategy? I don't want to buy a new bike every time he grows an inch. He is 59 inches now.

So you are in the range of XS/S adult bikes. If my daughter were growing that fast, I would be looking for a used bike that she could ride for several years and send on its way. I'd watch for an old steel frame 26" Trek or Specialized (Hardrocks are pretty common) and go from there.

Should we intentionally buy a bike that is too big? I'm thinking to get him a bike now, another when he is 13, and then at about 16, he will be full adult size. Or will we have to replace it more often than that?

That's your call - growing kids always present a challenge. My daughter grew into all three bikes I purchased for her but they were not TOO big. I'd start with inseam/standover and go from there. Finding a local bike shop would be also be wise.

For conversation sake - Looking at Trek sizes, the overlap between kids 26" Wahoo and the XS/S adult FX1 is intersting. It appears the kids 26 is more flexible with size adjustments. It definietly has a lower standover than the FX. Check out the reviews as a few people say the sizing chart is not accurate and their kids were too big.

https://www.trekbikes.com/us/en_US/b...colorCode=blue

My daughter was 10yrs old, almost the same height back in 2019 when she transitioned to a Scott Speedster JR24 road bike. We made a mistake in first buying her a 18" kids bike that she literally rode for no more than two weeks because she wanted a "faster bike like dad's" which ended up being the Scott. She's about 5'4 now and has been riding a women's Felt size S for almost two years now.

Absolutely.
I have a steel 12 in frame rockhopper sitting right here. They are out there.
Check out and maybe post an WTB ad on your local craigslist/offerup and maybe the neighboring county if you don't mind driving.

I bought this bike from an old woman, she bought it in Washington State, moved it with her here to the middle of Bumfukegypt.
It was 120 bucks and had been listed for a few weeks
A 12.
Just look and be patient

My son is 11, but a small 11, he was coming from a Specialized Riprock 20 (the previous design with suspension fork and tighter geometry), and so we initially went into LBS to look at 24" bikes. He is only about 54" tall, so we weren't sure he was ready for a full size bike. Happened they had a leftover Trek Marlin 5 in an XS, so 27.5" wheels, pretty dropped top tube. It's a little long for him, but that is perfect to give him room to grow into for a couple of years. Has mounts for a rack, and a nice integrated kick stand mount out back. So that will be nice for riding to school next year.

New or used I'd definitely try for a smaller adult bike than a bigger kids bike, I think it gives them more room to grow into. I remember my parents did something similar many decades ago, I graduated from a BMX bike to a Mongoose Supergoose, and rode that for a lot of years until late in high school when I got a Cannondale R400 road bike.

Get him on a small 26 and a 24 in bike. let him ride it and try it out. Last summer, my son was on a 16. He was too big for it. Got an old 20 in giant from a friend, thought it was perfect. discovered after two weeks my son wasn't riding it and was taking his little brothers 12 in bike. the pedals and seat fit him perfect. His arm were fully outstretched! he couldn't turn properly so he wouldn't ride it. Got him an 18 Royal Baby off FB and he road it all summer! Just got him a Gary fisher 20in with gears that he is tearing it up on and I can't wait til the 4th to go to my old MTB place and give him his first taste of single tracks!
If the bike is too big for him, he won't ride it and he could loose all interest in riding.

Bought when kid was 9.5yo about 15 months ago, took this pic 2 months ago and just had the raise the seat after this week's ride, not certain it'll make till the kid is 13. Went with a Cannondale xs 27.5 women's bike, the women's has a slight bend in the top tube that allowed for a little more standover which was very necessary when we bought it. Tried trek and giant and the kid couldn't clear the top tubes flat footed on their smallest 27.5" mountain bikes. My soon to be 13yo is on a size small cannondale f400 like the one someone else posted and she towers on it at 5'1" (she's also short for her age) and it will need to be replaced this year though really I'm gonna make her squeeze out the summer on it and pay or something much nicer. So I'd shoot for a 27.5" wheel for the better ride quality and see if a small couldn't be squeezed but go with an XS if you have to. Better to have them enjoy riding with you and replace it sooner than get too big and turn them off of riding.

I went to the local bike shop and got my 10 year-old daughter a small used Trek mountain bike. It must've been their smallest model with the 26" wheels. It's a quality bike, tuned up by the shop. Right now, she's 4'11 and the seat is all the way down. I think I spent $150; a lot compared to a Walmart bike, but not much compared to a new Trek!

It's just a bit big, but she can handle it and it will work for several years. She's 11 now and it's fine. My 17 year old (5'5) still rides her bike with the same size frame; she's not big on full leg extension anyhow; likes the seat pretty low so she can touch both toes down at stops.

Before this, I bought my daughter a Trek BMX type bike, but she outgrew it FAST and it was wasted money.

My 11 year-old daughter now prefers to ride my folding electric bike. It adjusts for a HUGE range of rider sizes. With the seat and handlebars all the way down, it's fine for her. With them up quite a bit further, it's fine for me.