I am 20 years old and my sister is 2.5 years younger than me, but the developmental age gap is much higher. This is harder to put a number on! She is neurodivergent with a spiky profile, so her developmental age varies on the day and by the task. She also has some minor health conditions that she just needs help managing the sensory side of.
When I was aged 14-17, we used to enjoy going into town together. She liked being taken shopping and going on the train and I liked there being some structure to this trip that made it easier to spend time with her. My parents would help by taking us to the station, so we did not have to get the bus there, which would have lengthened the trip considerably.
I think there being some structure was the most important factor for this being a success – I suppose this could be applied to lots of things. We would plan it advance and choose the trains so we knew how long it would be and pick a few key shops to go to, so there was a rough plan. The novelty was also important, as this was not a journey she could do independently. If she went alone, she would be very scared of the train and not go at all. She would also be overwhelmed in the city centre.
I felt responsible on these trips and was definitely acting as the adult, but it was a nice thing for us to do together, as I enjoyed it too and got something out of it. Other things together e.g. certain games were much more one-sided!