Is it safe for adults/kids to take 1/2 teaspoon homemade honey elderberry syrup per day as a preventative measure? should it only be taken when ill?

Not for babies!: No baby, 12 months or under should have any honey at all due to infant botulism. There have been 2 VERY SMALL studies on elderberry extract, none for honey. Results to duplicate even these small studies need to use the exact form of elderberry used in that study, if trying to get same results. Supplements are not regulated and so there is no guarantee on that amount or useability of the form used