I work in tech and plenty of people wear backpacks here. Both men and women.

A plain and simple backpack doesn't look childish to me.

I have been wearing crossbody, and now that I need to carry both my laptop and a glass lunch container and a water bottle since our new temp office has no storage place, a backpack has been on the shopping list.

This looks absolutely amazing to me and not childish at all. But I am not sure the one shoulder sling is not going to overload the shoulder like the crossbody bag does. Plus I am not sure of the space it has.

Also look at stuff like this one, which also doesn't come off as childish at all.