How to use R, where is my data?

[MaxManus]

I'm using R and used the command

cod<-read.table("http://www.myweb.com/data.dat",header=T,colClasses=c("numeric","factor","factor","numeric"))

How do I use my data?

I want to plot age against length

Edit:
Solved
The variables names were
data$age
data$length

and not x1 and xt as I thought

plot(data$age,data$length)