The way people should look at it is focus first on what development season the prospect is in, and then after figuring that out consider their relative age compared to the other prospects in the same development season.
Normally this works by looking at the player's birth year. Using this year's crop as an example, most players were born in 2006, but there are some 2005-borns with late birthdays that are also first-time draft eligibles. While they are drafted at the same time, the '05 borns (for the most part) are a season ahead of their '06 born peers.
Using a couple players as examples, Alexis Lafreniere and Nathan Legare were friends and played together for two seasons for Mille-Îles Seigneurs Bantam AAA in 14/15 and 15/16, plus a season of Midget AAAA for the Saint-Eustache Vikings in 16/17. After that season both were drafted to QMJHL teams because both were '01 borns. They then each played two QMJHL seasons in 17/18 and 18/19, but because Legare was born earlier in the same year as Lafreniere, he was eligible for the 2019 NHL Draft, while Lafreniere had to play an extra season prior to being drafted in 2020.
Even though Lafreniere was 6 months younger than Legare, it would be silly to think he was a year behind Legare in development when he got drafted the following year in 2020. Both had the same number of seasons played at the same levels since they were 14, but because of the Sep 15 cutoff put in place to ensure NHL teams aren't employing legal minors, Lafreniere got an extra season to develop and impress the scouts before they could draft him. But obviously Lafreniere's production was most accurately compared to other '01 borns like Legare who are in the same development season. Still most people on this site that year were comparing Lafreniere's stats in his 3rd CHL season to an '02 born Byfield's 2nd CHL season, which provides a slanted view of their potential and upside in favour of the late-birthday player who is a season ahead.
Once you factor in that '05 late-birthdays like Demidov, Buium, Yakemchuk, etc... are a season ahead of their '06 born fellow draft eligibles, it becomes obvious why their numbers are often more impressive, and why they are often favorites of unaware casual scouts on HF.
After you consider the number of development years a player has under his belt pre-draft, by all means you can then factor in how many months younger/older they are compared to other prospects. I personally don't put a ton of weight into whether a prospect is born Jan 1 or Sep 14 in the same year, but studies to suggest it plays somewhat of a role. Obviously the slightly younger prospect is slighty less physically and mentally developed, even if they have the same amount of hockey season splayed.
It's also important to factor in if a player is a late comer to competitive hockey. Some prospects don't start playing until later and that obviously means they have less development time as other prospects who started early. These players are the exception to the rule, however, and I certainly don't think it matters a ton if a player was on skates at 5 or 7, for example. Still it's something to take into consideration.
I think this is the biggest blindspot for most of HF when evaluating draft eligibles. Shocking how many people legitimately don't even consider this when comparing prospects.