HO and N scale trains have a suggested age of 14. If your child is less than 14, I suggest you only allow him to use it with close adult supervision. If your child is below 10, you might find HO scale is too delicate for him. O scale and Large (G) scale are more durable and lend themselves more to younger children.
I had a "scale" Hornby Dublo railway from when I was 12 or 13 years of age.
That's "scale", as in flex track, No. 6 switches, an around the walls (in my parent's attic) prototype based track plan complete with timetable etc., so age isn't the limiting factor.
I knew a couple of boys in my school who were scratch building flying aircraft from balsa wood at that age as well, much more impressive skills that building a model railway.

All a younger child needs is encouragement and guidance and my dad wasn't even interested in trains but that's what he gave me.
Cheers
Roger T.