mod <- gam(bodyMass ~ s(Age, bs = "cr"), data = mydata, method = "REML")

pred <- with(mydata, data.frame(Age = seq(min(Age), max(Age), length = 100)))
pred <- transform(pred,
                  bodyMass = predict(mod, newdata = pred))
plot(bodyMass ~ Age, data = mydata)
lines(bodyMass ~ Age, data = pred, col = "red", lwd = 2)