Singapore’s Age limits

I’ll like to hear the rationale for the following contradictions :

 

21 years old age limit for R(21) movies

 

21 yrs old age limit for voting

 

18 yr old age limit to drive

 

16 yr old age limit for killing someone or being killed in war (NS age limit)
 (you can’t vote for the govt you want, even tho’ the govt who comes to power has the authority to send you to war to die)

 

16 yr old age limit for age of consent to sex
 ( i can’t understand how people think that we should not lower the R(21) movie age limit, should not let our 16 year olds learn about sex in sch, do not want our 16 year olds having sex, but shoud lower the 16 yr old age of consent)

 

16 yr old age limit for sentencing someone to death for drug crimes
(http://www.yawningbread.org/arch_2009/yax-1074.htm)

 

 

14 yr old age limit for being given to marriage legally
(even tho’ they have no capacity to vote their govt, or to handle a R(21) movie, they can be allowed to make a lifetime commitment , and have kids at 15yr old??)

 

Explain this to me, someone, please!