We... can't tell just by looking at them
You can observe their behaviors and google it, or you could get your bird tested
Usually, when you buy your birds, they have a leg band with letters and numbers on it. The breeders should already have your birds tested... I think... and have the data of the bird.
They look like two hens to me judging by the cere colour
I better mention they both, especially the blue, have badly overgrown beaks. Stick some cuttlefish bone in with them, if they don’t use it you’ll need to get their beaks trimmed by an avian vet
Well, the blue and yellow one is over 4 months old, I can say that for sure, as is the other one. You can tell because their forehead bars dont go down to their cere. They both look like hens, but they might be just a bit too young to tell.
Fellows here are right on spot! First the turquoise needs an urgent beak trim that must be done by a veterinary. Look, sometimes beaks overgrown due to poor diet that might affect the liver functioning. Of course changing the diet will help your bird to renew the beak to a more healthy status. Beaks as our finger nails grow all the time and after the health of your bird is reestablished the beak will grow a nice new layer from the nostrils toward the tip.
Please check with a vet for advice regarding the beak issues.
Regarding the ages, I would say both have less than one year. Older budgies will have lost their forehead stripes (like in the picture) as mentioned by Stormypica, still they are not that old as mature budgies? eyes tend to have a white ring around the pupill.
Considering they are both less than one year old, I would say they are not sexually mature yet. For that reason, the cere colour of their noses is pinkish. It is common for young males to have it pinkish before it turns blue, I think the turquoise is a male. However it is not that easy for lutino, albino or pied mutations. The yellow one is a pied mutation, which means the bird has only partially the expected markings. In such mutations, both male and female can keep a pinkish cere their whole life, however if you observe the nostril region of the yellow bird you can notice it is a bit paler than the turquoise. In that case, I would say it is a hen as the fellows here pointed out.
Literally its like 7 dollars to send a feather or blood or anything to Florida and they will give you a DNA analysis. And can check for other bad things. It's less than a McDonalds combo meal and you get the results in like a week or less.