We just follow the interests as chances are by the time DS4 is 'older' his interests will have changed. His love of space started at 2 as well. We went to the Observatory for a present for his 3rd birthday. He loved it. At 4 he remains obsessed with space but in a more in depth way but I know will love going to the Observatory again. There will be plenty of 'firsts' left I'm sure.
Enjoy the journey!