We just got awarded higher rate mobility for dd2 (hooray) but not until the summer. When I rang up to question this, it is because it was a renewal of her existing DLA claim rather than a new claim. The existing claim runs out in the summer.
We should have claimed within 28 days of her 3rd birthday apparently - aagh! (she was 3 last May). The woman at DLA told me that they couldn't 'force people to read the forms'
Just thought I'd mention this so that anyone with a child coming up to age 3 knows the situation!