test <- read.table(textConnection("age popperc
0-5 7
5-15 14
15-20 7
20-25 7
25-35 7
25-30 7
30-35 7
35-45 15
45-55 14
55-65 10
65-75 6
75-85 6"),header=TRUE,stringsAsFactors=FALSE)

midval <- sapply(strsplit(test$age,"-"),function(x) mean(as.numeric(x)))
breakval <- strsplit(test$age,"-")
breakval <- as.numeric(c(sapply(breakval,head,1),tail(unlist(breakval),1)))
hist(rep(midval,test$popperc),breaks=breakval)