library(lme4)
library(nlme)
lmer(height ~ age + (age | Subject), Oxboys) #warning

mean(Oxboys$age)
Oxboys$age2 <- Oxboys$age - mean(Oxboys$age)
lmer(height ~ age2 + (age2|Subject), Oxboys)  #no warning


lmer(distance ~ age + (age |Subject), Orthodont) #warning

#But a more complicated mode:
lmer(distance ~ age*Sex + (age|Subject), Orthodont)
