Quantum Tunnel

I believe the real reason captain america was old after putting the stones back and living with Peggy, is because the quantum tunnel that he used was more different and advanced than the other ones we've seen so far. Other than just traveling to alternate past timelines, it also grants the user access to travel to the past of their own main timeline, allowing more linear time travel, thus explaining why Steve was old in the present. Also in doing that, there might actually be two captain americas depending on if Steve traveled to a point in which his past self was still trapped in ice. Otherwise we wouldn't have been able to see old Steve in the first place and he would be in an entirely different timeline. This might even explain Chris Evans' apparent return to the MCU. They might explore both old Steve and the Steve that was still stuck in the ice and bring him out.