Can I put two axolotls together in a 20 gallon tank and if I can, is it ok if one’s a juvenile?
General rule of thumb is 20 gallon minimum for ONE axolotl. Each axolotl after that requires at least another 10 gallons up in size.
Since you mentioned a difference in age, please be aware that a size difference of approximately 2 inches or more is fairly likely to result in bullying, food hoarding, and potentially cannibalism, even among adults.
Additionally, he decision of putting two axolotls together is dependent on their individual personalities. Some axolotls hate being in a tank with others, some are fine with it. Even if it works out well, you should always have a backup solution if it doesn't work out for any reason.