So far as I can remenber, his parents always took him to some events, most of them were National related and visits from another countries. Because he was born a Crown Prince, he's been trained since birth. As for solo engagements, I can't recall the exact age and year, but it's been a while since he does solo engagements, I would say probably around 4/3 years by now? The first solo engagements he took were all school related, so 4/3 is plausible.
As for Lalla Khadija, she's a little girl, there's no need for her to do engagements, however she can make company to mommy once in a while to her foundation events, but that's only that and rare.
As she turns 18 years old, she might undertook some solo engagements as her aunts and mother do. So we have to wait a bit longer for that
That's why I was so sad this year we didn't get any picture on her birthday. We don't see her often, and that's why I'm always excited for her birthday pic. Hope they keep the tradition and let this year be an exception.