Here's why Scott Lang and Janet van Dyne both aged differently in the Scott was left in there in the post-credits.
These events have significantly shaped both of their stories. Janet entered the Quantum Realm in 1987 after going subatomic to stop a missile. Due to her continued shrinking, she was lost to the realm for over thirty years and managed to survive until Hank Pym found a way to rescue her. Not long after Janet's rescue, Scott entered the Quantum Realm on purpose to collect Quantum Particles to help Ghost. Instead of the Ant-Man team pulling him out, he was trapped in there for five years due to Thanos' snap. Even though both characters spent a lot of time in the Quantum Realm, their experiences were not the same.
When Janet returned from the Quantum Realm after 30 years, her physical appearance confirmed that she aged at what appears to be a normal rate. This would indicate that Janet experienced time move at the same rate as normal. However, Scott revealed in complexity of the Quantum Realm, as Marvel Studios head Kevin Feige has said in the past that there are "various levels," so the concept of time appears to be different depending on which part of the Quantum Realm one is in.
Due to the vague details about the Quantum Realm, the comparison of their adventures is difficult. Janet mentioned in Ant-Man and The Wasp that her current location was in the Wastelands beyond the Quantum Void. It didn't appear that this is where Scott went to get Quantum Particles, but it also isn't confirmed what part of the Quantum Realm he did visit instead. Still, since Scott and Janet were stuck in different parts of the Quantum Realm, then their individual levels having unique effects on time can be used to explain Janet aging and Scott not.
There will likely be further explorations of the Quantum Realm in future MCU movies too, which could add more clarity to this situation. Franchise producer Stephen Broussard already said that Ant-Man 3 isn't known yet, but it is just one of a few future MCU movies that could utilize the Quantum Realm.
