Even kids 7 and up could do this trail with adult supervision
It is important to be aware that the longer Monkey Mountain Trail on this website does not take you to Monkey Mountain, merely connects you to this trail, which does reach the summit.
The trail starts off nice and easy as you'll be hiking on the Monkey Mountain Trail for the initial 1.5km or so. The trail to the summit is marked with a small cairn. From here it gets steeper. I suspect it would be quite slippery in the rainy season as it's mostly bare ground that you're hiking on. Near the top there is a bit of scrambling to gain the summit. There are very nice 270-degree views from Punta Mita to Sayulita. This is a good afternoon or early morning summit, as the light is nice for photography.