I get lots of requests to make cards for special age birthdays and my favourite way of doing this is by focussing on the number itself. I make the numbers using Word Art, reverse the image then print them onto the back of the paper. It's easy to carefully cut round the printed lines - I tend to use a pair of fine embroidery scissors for this task.
The backing paper is by Nancy Watt for making Cards Magazine and the stars are made using Sissix die-cuts. I've entered this card into the 'Milestone Birthdays' challenge at Crafty Emma's Store CES Challenge