As soon as possible.
In older games, certain Pokemon that evolved from evolution stones like Arcanine had limited movepools of moves that could only be learned at level 1, meaning you were forced to go to the Move Reminder in order to get these moves reteached. Additionally, these movepools were extremely limited, meaning that if you evolved too early, you would miss out on important moves that you either didn't have access to anymore, or you had to use a TM to learn the move. An example is Gen 3 Growlithe. Since it learns Flamethrower at level 49, if you waited to evolve your Growlithe until then, you wouldn't have to worry about wasting a TM on it.
However, the newer games improved on this a lot, with features like unbreakable TMs, TRs, easily accessible move tutors, and free move reminders, with a couple games backtracking a little on this (Cough BDSP Cough). This means that you can evolve a Pokemon sooner without having to worry about wasting a TM, and in the Gen 8 games, stone-evolution mons had their movepools changed so they still learned the same moves as their pre-evolution, so you wouldn't have to worry about losing access to a move.
In the case of D(r)ipplin, though, you want to evolve Applin right away. Not only is Applin very weak, with base 40 offenses, but its movepool is atrocious, only learning Astonish and Withdraw via level up, Pounce and Tera Blast via TM, and Defense Curl, Recycle, Rollout, and Sucker Punch via egg moves. The game is encouraging you to evolve your Applin, and it's definitely worth it, as Dipplin's movepool and stats are FAR better than Applin's. Also, you can easily remember moves by going to your Pokemon's summary. To evolve Applin, you need to use a Syrupy Apple, which you can obtain at the Mossfell Confluence for 500 Pokedollars, according to Serebii.
Hope this helped! And if for some reason, you do want to wait until level 73 to evolve your Applin, be my guest, nobody is judging you... except me, of course ;)
