My oldest two dds are v close in age - 21 months - and have always been good friends. But now the gap seems huge as dd1 is 11 and has started secondary school. Dd2 is nearly 10 and still at the playing with toys stage but she's starting to miss her big sister's company.
Tonight dd1 had a call from a friend at school, they're going to meet up at the weekend for a bit of shopping. Dd's never done this before and is excited, as am I for her. Dd2 overheard a bit of a conversation and said to me 'I'm losing dd1' 
I explained to her that at the moment it's a big gap but it won't be long before they're both teenagers together.
Anyone else experienced this? I also have a dd3 (nearly 6) but tbh her and dd2 often clash like mad and dd3 can be quite naughty so not a good playmate for dd2.