For fm2
(linear mixed model), you can do
sqrt(diag(summary(fm2)$varFix))
#(Intercept) age SexFemale
# 0.83392247 0.06160592 0.76141685
For fm2.lm
(linear model), you can do
summary(fm2.lm)$coefficients[, "Std. Error"]
#(Intercept) age SexFemale
# 1.11220946 0.09775895 0.44488623