We know the readership of Jump as a magazine, with 25.8% being between ages 19-24, 27.4% 25 and above, 17.6% 16-18, 16.4% 13-15, 9.6% 10-12 and the remaining for 9 and under. I think the target the magazine as a whole is aimed at is the 13-17 or that mid-middle school, high school age group. Stone was said to be popular among younger readers though I don't know the specific numbers. I saw a report saying that a lot of elementary schoolers bought Heroaca Vol. 15 once and a big portion of its viewership/readership are middle and high schoolers. OP is a weird case. Its got a sizable kids TV rating but an even bigger Men and Women aged 35-49 rating. For example, this is the transition from Kitaro to OP for the week of 5/7-5/13 :
*5.7 *3.3 *8.0 *0.6 *3.5 *2.7 *2.4 *3.6 *5.3 *2.4 ゲゲゲ
*6.3 *3.4 *4.8 *1.5 *2.9 *5.8 *2.5 *5.2 *6.5 *1.2 ワンピ
The bolded are in order, Kids rating, Men 35-49 and Women 35-49. The actual household rating you saw on Video research is 6.3 for OP and 5.7 for Kitaro. This could simply mean that a lot of parents are watching these shows with their kids but it doesn't always hold. Nichiasa is also a thing so some kids go there. Even then the older people rating doesn't drop.