At birth. Bring them to the Traditional Latin Mass. Sing them Gregorian Chant lullabies.
In first grade. Include Latin pieces in their memory work. Sing the Latin hymns in the Parish Book of Chant together as a family throughout the Liturgical Year. These children are just learning to read English, so if you want to use a Latin curriculum at this age, try a very early-elementary curriculum/video series (such as Song School Latin, perhaps) to keep it easy, light, and fun.
In third grade. I love teaching this age, and it’s around 3rd to 5th grade when most people think of starting children with a Latin curriculum. I recommend starting this younger end of the age group with a basic, age-appropriate Grammar/Translation Method textbook, of which there are many, among them worthy selections from Memoria Press.
In fifth grade. At this age I believe an excellent Grammar/Translation Method textbook is still a great idea. Henle Latin comes to mind of course for Catholic students. But even a Classical Latin book, if very good, it will get students on the road to Church Latin. Such a one is this public-domain Elementary Latin. (I personally LOVE that particular author!). As I’ve written before, later on in their Latin studies they should have a Natural Method series to finish off (see below for more on that).
In high school. For those starting Latin for the first time in High School, I recommend easing in gradually to this inflected language with a very basic guide, and then following it with the LNM Series (or Lingua Latina by Orberg for Classical Latin). Latin musical selections should be among their repertoire choices, too.
It’s never too early for Latin, and it’s never too late. Learn it, pray it, sing it, love it!