@ashmarie Coming to one of the parenting subreddits and asking here first before you address your neighbors was and will continue to be the right thing to do first. Good on you and thanks for coming and asking.
I suspect your intuition on age is correct, given they're speaking basic words. If it's regular, like practically every night, it sounds pretty reasonable and normal. That a kid wakes up and screams bloody murder is not itself cause for alarm, that's just how kids cry. Or can cry. And crying is the biggest tool in their communication toolbox. A child's temperament can be drastically different from one to the next. So many parents struggle with fussy children, in one form or another; typically the most brutal would be in regard to fussy eating, sleeping, or hitting/biting/some social behavior issues they need to learn or age out of. Mercifully, my boy is AWESOME in these regards; others - well, not so much.
There's nothing you can or need to do, nothing you can or need to say. They know. They 100% know. They're probably frustrated parents in this capacity, and would probably be embarrassed that you brought it up to them that they've made it a problem for you, too - and don't take this next part the wrong way, they would probably get angry and defensive with you because of a, "you're not a parent/the parent of my child, what do you know" sort of thing. They are trying and don't need to entertain someone's opinion that they're not doing enough, or the right things - and no one invited you.
You risk brushing up on the wrong nerves and coming off as insensitive.
The best advice I can give you is, frankly, what you're already doing. Tough it out. The child will grow up and grow out of this; it may take a few months. To me, that's essentially no time at all; to others, that may seem like a daunting burden, and I can only be sympathetic.
So I recommend you keep up on the earplugs, and if you want to build a bond with your neighbors, especially if you think they may be cool, leave them a tin of cookies and a note with some encouraging and supportive words, that they're doing a good job. Maybe suggest a board game libation night and make friends. Being a parent is a lonely gig at this stage because the child and family demand so much time, and having another adult around on the occasion is just... Nice... And building that rapport will develop you some leeway, if necessary, because now you're not just a faceless neighbor from downstairs.