A decrease in muscle strength as a result of a reduction in physical activity - such as two years more or less in lockdown - can make symptoms far worse/appear noticeably for the first time, especially when combined with growth spurts.
Straight after the first lockdown, it was shocking how many kids came back not just thinking they were going to die if they got out of breath, but unable to stand straight, sit straight or walk without their feet/ankles/shins/legs/hips/back hurting or popping, worsened by the sort of shoes some of them were wearing (no support, single straps, no cushioning).
There's a good chance that your DD overpronates - her foot collapses inwards under weight - which puts too much of a stretch into weakened muscles, tendons and ligaments.
Something that does help whilst muscles in the core, upper thighs and lower leg/foot is getting arch supports/orthoses - custom ones would be the best and, as you have been able to pay for a private consultation, perhaps it would be something else you could purchase?
Add in the exercises you'll be told about, good trainers (ones where the insole comes out so you can put the orthotics in) and good shoes (again, where you can put in a separate set of orthotics - they never remember to change them over after PE) which may need to be a size larger to allow for the loss of space - and she will be able to at least strengthen and stabilise her joints more.
tl;dr It does start at age 12, but it can become noticeable for the first time at that age, especially when you add lockdowns into the equation.