The lack of resources generally in the past was a major hurdle. These days it's all there for them. That said, not having them it did teach valuable life skills such as resourcefulness and resilience.
Someone earlier in this thread asked what suggestions do people have to get their kids to a competitive JMC level. My answer would be apart from looking at a few past papers to get familiarity with the questioning dont specifically study for it.
In my opinion get your kid on a maths app early and gently encourage them (not bribing). Try and make it a shared problem, get involved. With my DD on Doodlemaths I setup a spreadsheet, one column her actual age, the next her doodlemaths age, and showed her the progression over time. This was motivational for her to see progress she'd made. Start off maybe 1 level or 10 questions a day, when they finish ask them to try 2, perhaps 3 levels. In no time they are doing 10+ levels per day, trying to keep a streak, are fully engaged and it's easy for the parent. They kid takes responsibility for it all. These apps get harder gradually so you never really have to do any teaching.
Don't care about accuracy, if your kid's doing 100+ questions a day they learn from their mistakes automatically.
For a few years now my DD has just been coasting really, she does the bare minimum for maths apart from finishing early vs the rest in class and moving onto harder stuff. There is no extension work at home at all. It's all very economical.
I reckon anyone out there, just in the summer hols with the right approach can advance their kid 1yr in maths. It may be a tough start but once it gets rolling things can move very very quickly.