At what age did your oldest really start interacting with the younger baby? Right nOw he's just in that stage where some times he thinks she's fascinating but most of the time he ignores her until she cries... Then he cries. He does occasionally crawl over and try to give her his toys.
Re: If you had 2u1
Mine are 14 months apart, so not quite as close as yours... My DD didn't pay attention to DS AT ALL for the first few months. She would act out a bit if I was feeding the baby and not paying attention to her. Honestly, it wasn't until a few months ago did she start really paying attention to DS... and they really just started playing together.
I don't remember because I had 2U1 and those two years or so are a blur.
I would have to guess that went the younger one started sitting up (and taking his stuff) is when he started paying attention. So maybe six months or so.
At 6 months when the baby started crawling/playing with toys more. Before then, he was like your son--ignored her until she cries and occasionally gave her toys.