When to use candies in let's go pikachu

Candies always give you +1 to the associated stat (Or +1 to all stats for species candies.) Apparently the candies increase the "Awakening value" for each stat, and the cap is 200 for each of the six stats (They're independent so you can get up to +200 points in every one.)

To actually answer your question, no, it doesn't matter. The stat boost for candies will be the same at level 1 as level 100.

You can get a more info for this here.

It doesn't matter

You can use the candies whenever you want, from my own experience I can say is encouraged to use them at lower levels so your Pokemons get stronger, there is no penalty from having a low-level Pokemon with +200 in each stat, because the game is oriented to younger and/or newer Pokemon players, so this mechanic was implemented to help you out when you feel your party is weak or you want them to be super strong.

In YouTube and forums you can find people talking about beating master trainers with low-leveled Pokemon with the use of candies.

This is for in-game play-through, in case you are talking about online play is somewhat different, because there are two modes to play, Normal and Anything goes, if you plan to only play Normal, all your Pokemon will be at Lv.50 with no AV (values added by the candeis), but if you want to play in anything goes, make sure your Pokemon are at Lv.100 and have +200 stats because they will battle at the level they are.