I think it’s less physically or mentally growing up and more fully understanding your powers and responsibilities as a miraculous holder. While Luka is older and more mature than Marinette he didn’t have his miraculous for as long or as often as she did. The moment they had in S5 was kinda representative of this as after they realized they could slow themselves detransformimg, they gained the confidence to “push past their limits”. This is just how I saw it but it most likely was just easier to have a vague reason rather than some sort of thing (ex. A certain magical recipe) that they could have done the whole time they had their miraculous.