It is generally accepted that younger birds are more adaptable to changes, but it is not 100% necessary to introduce a new bird when both are young. My moms amazons had not seen a new bird in almost 40 years when they met my amazon (who was around 10/11 and likely hadn't seen his own kind since he was a baby). They were very curious about the younger new guy who came for occasional visits and accepted him right away (we don't live nearby anymore or they'd still be having occasional visits with each other). There is also never any guarantees birds will like each other, even if bought around the same time. My parents got their birds in a few year span, and the female amazon and male cockatoo don't get along. Each has amputated a talon/toe tip from the other over the years and they'd seriously injure or kill each other if allowed out together.
Yes, parrots do go through puberty (larger species start between 4-8 years old) and yes there can be some issues there, but generally speaking the biggest issue with parrot puberty is unsuspecting owners who have no clue it's coming, why their bird is acting out or how to help their bird through it. Being aware and prepared helps a lot whether you have 1, 2 or 10 birds going through it. I mean, people don't wait 18 years between children so they don't have 2 kids going through puberty/teen years at the same time, do they (and parrots can't even drive or want to stay out all night
)? Puberty in parrots, like puberty in humans, only last a few years and some parrots don't even get it that bad (though there's no way to tell until it happens how your individual bird will react to the onset of hormones).
IMO, the biggest thing to consider is that your #1 reason for wanting a second macaw is that YOU want a second macaw, not as a buddy for your first macaw. Second factor, of course, is if you have the time for both if they should end up not getting along and need to be out separately? Lastly (and I'm assuming you can) is if you're financially able to take on another large parrot- proper diet, vet bills, toys, initial setup... all that stuff.
