I think because of age they maybe testing you same as a toddler (I have a three year old and it’s crazy how some bird behaviors are similar)
I have a cockatiel I hand fed and she has never bit anyone but started to scream and put her beak against our skin like flapping angry acting like she will bite whenever we put our hands down near her and she doesn’t want to step up or go back in her cage. She’s over a year old and we just accept that she knows what she likes and doesn’t. She acts the same way with our other birds if they do something she doesn’t like (like touch her toys). Our birds all have their own cages because of it. I think for her it’s a personality thing and she’s a bit bossy. We give her space when she wants it and give her attention when she asks(chirping to come out and walks over to us). We already know her body language. I have her out but the moment she starts her bird rants we put her back. We try not to give in when she acts that way but we also realize if she doesn’t want us to touch her at times then do so when she wants us to. There’s times when people want led to hug me and I didn’t feel like it so I understand.
It’s different with our Double yellow headed Amazon since he’s a larger bird. He’s almost 7 months we got him at 6 weeks and head fed him, (recently fully weaned) This week his new thing is to begin to put his beak around our wrists or added pressure to my finger, which Is when he refuses to step down (I only let him stand on my wrists or hands, I avoid shoulders). Whenever he does this new thing I say “ow” not too loudly since loud sounds they interpret as a good to thing, but in a stern voice (same one I use with my kids when they act up)And I put him up on one of the play stands and ignore him for a while. I read that putting them back in cage as punishment is way to teach them not to bite, but I worry that they will bite to go back. So I just ignore them when they do this, it seems to work. But I also keep in mind that we can’t force them to do anything they don’t want to do. Figuring out what they love like a treat or toy is great. Clicker training is great for this also. It takes time to find out still. Only just found out ours loves peanuts so never feed them to him only for treats to reward good behavior. Then again I’m still learning what works for him and what doesn’t.
I do think like human children they go through stages and right now yours are learning what they can do and get away with. They might not even realize it hurts you either. Some birds play rough also, half the time I can’t tell if our budgies are playing or fighting. Which is something to also keep in mind. Study how they act with each other and focus on learning their own individual body language and sounds. Each bird is different and express themselves differently. I tried to use guides to go by on reading their body language but it’s not always the same for each. Some things are but lots are not. Eventually you will understand the way they speak to you with their sounds and body language.
Avoid shoulders! I keep telling my family not to have them on theirs cause of bites even unintentional.
Hope you find out what works for you and them!