Why weren't the cast of Better Call Saul aged down?

I'd go for "willing suspension of disbelief".

They definitely made Jimmy/Saul look younger for the flashback scenes when he's running con-tricks as Slippin' Jimmy, and they distincly aged him for the flash-forward scenes as Gene Takovic.

I think we're just supposed to accept that without several million dollars'-worth of CGI in each episode, the actors are going to look older. Breaking Bad was first released in 2008, 12 years ago.

Besides what Tetsujin points out about willing suspension of disbelief, it also seems like de-aging is what they were going for with some of the hairstyles. Here's Saul Goodman in Breaking Bad:

And here's 5-10 year older Bob Odenkirk playing his younger self in Better Call Saul:

Pretty successful, in my opinion! My guess is that they went with this method instead of using makeup or CGI for everyone because of the sheer expense, even if it meant they couldn't do it with characters with less hair, like Mike or Gus:

Finally, the show doesn't take place that much earlier than Breaking Bad. BCS takes place in the early-to-mid 2000s, while Breaking Bad takes place in the late 2000s-early 2010s. This gives it some wiggle room for plausibility.

I've rewatched some clips and I propose this:

We see Gene Takovic being trapped by doors closing on him in the trash compartment. (How's that for a metaphor?) And after he waited on the floor, we see him leaving, but also a scratching on the wall:

SG was here

That moment, between when he sat on the floor, is what we see in Better Call Saul. He is reminiscing on what led him where he is now and the man he was. That's why he has the Saul age through series (with that haircut) and people he meet for the first time are the age he last remembers them.

Proof of theory: When we see Jimmy McGill showing his brother Chuck he passed bar exam, they are not younger. Chuck has the same gray hair and same haircut as when Jimmy sees him for the last time. Also Jimmy looks exactly the same (with haircut) apart from clothes for a much younger, more frivolous person. Clothes that are in, for me, a clear contrast with ambulance-chaser haircut.

Semiproof of theory: If this is in Saul's memory, how does he know about things that happened without his presence? Because he was told about them by participants. That's why people complain that BCS Mike is a much better/smarter person who shouldn't be shot in BB. But this is how Mike described himself to Saul. In BCS Mike is this almost superhuman comic book detective. It's because it's his stories and in such everyone like to paint themselves better, smarter, with all the coolness and strategy all laid out before acting on them.

Another semiproof: There is a scene when Jimmy is sitting outside Chuck house who scolds him. This is the same position he sits in that trash room. But during that scolding Jimmy is becoming Saul. And until one point it was Saul who was playing Gene. But when he stands up back again, he might stand up as Gene. "SG WAS here".