Thats the QR timeline. After he grow old with peggy, he went back to reality timeline n sit on that bench

Steve can use GPS thingy to set time n location to 2023 on that bench after grow old with Peggy in QR timeline

Because Quantum Physic. Possible to be in two locations at the same time—at least for an atom or a subatomic particle, such as an electron. For such tiny objects, the world is governed by a madhouse set of physical laws known as quantum mechanics.

So we must hold to this idea. 2 timeline only. Our timeline and Quantum timeline.

Janet wasn't wearing a QR suit was she?
Its possible she was experiencing what Ant-man did during their first test with Professor Hulk's prototype.

Thanos's chief scientist simply made more Pym Particles. Since he's a freaking genius and all that. 

"how old cap return?"

damn. now we talking.. russo plan to do some story telling for future project..

my theory is this..

A: 1950
B: 2023
C: 2066

if quantum open in 2023, it can close in few seconds.
from B, u cannot go C because that is future.
in order from B to go C, u need portal to open in C
but, u can go A because that is ur past.

lets follow steve in endgame.

from B, he jump to A
from A, he didnt return. he stays
if he stays until C, he can jump to B without portal.

the scene where steve jump from NY to 1970 prove this theory.