It's a complicated question and many SEO experts will give you an incorrect answer depending on how long they've been doing the job and also how much history they know about the industry. That's not their fault, just one of those things.
The Sandbox used to be a thing in the mid to late 00's. Back then it almost always took about 3 months for a brand new website to exit the box and begin to rank properly. Back in the day we would just factor in this time lag and simply conduct our SEO blind (not checking the SERPS, just knowing roughly the backlink metrics we kinda wanted to see). This was made easier by the fact you could just plug backlinks in with your anchor texts and "hey presto" you have a page 1 ranking.
Anyway, in the late 00's/early 10's (not sure exactly when and my dates could be a little off) Google decided to get rid of the sandbox. This was good news as it helped younger sites and especially small businesses get a foothold in the SERPS.
Unfortunately, by getting rid of the sandbox a loophole in ranking appeared. This is usually called the "UK payday loan" saga (infact a Google update is named after it) and goes a little like this.
Okay, so there's a little more to the technique than this and it's obviously something that's largely been patched by Google; however, it was a legit black hat technique and one I used myself to rank payday loan sites back in the day.
Anyway, in a post "payday update" world, whilst there's no "sandbox" as such, there is a heavy weight now towards established and aged sites. This is so pronounced that the trade in established/expired domains is still very profitable. You could argue that the rise in PBN sites is directly caused by it too.
TL;DR There was a sandbx back in the day which has been removed; however, the algorithm is now so bias towards established sites and against newer domains that it can "feel" as if the sandbox was back in use.