I'm afraid that figures aren't normally published for typical 11+ standardised score tables - but you can get an ideal of the typical range by looking at the Key Stage 2 Standardised Scores tables which are published by the QCA.
If you look at the Tables for 2005 key Stage 2 Written Maths and take an 11+ Pass Score of 115 (typical I believe for most 11+ exams), then the 115 Pass mark would range from 64/80 (80% Raw) to 69/80 (86% Raw) over a 12 Months Age cycle. So in this case for this key stage 2 exam a difference of 1 month in age was worth 0.5%extra or less of the raw score.
The QCA url is http://www.qca.org.uk/12422.html
Actual tables are printed out in the results downloads so you can see an example of variance.
Hope this helps.