Look up Datediff in books online:
Select
DateDiff(yy,Cast('10/21/1970' as DateTime),Getdate()) AgeYears,
DateDiff(mm,Cast('10/21/1970' as DateTime),Getdate()) AgeMonths,
DateDiff(dd,Cast('10/21/1970' as DateTime),Getdate()) AgeDays,
DateDiff(ww,Cast('10/21/1970' as DateTime),Getdate()) AgeWeeks,
DateDiff(hh,Cast('10/21/1970' as DateTime),Getdate()) AgeHours,
DateDiff(mi,Cast('10/21/1970' as DateTime),Getdate()) AgeMinutes,
DateDiff(ss,Cast('10/21/1970' as DateTime),Getdate()) AgeSeconds,
DateDiff(qq,Cast('10/21/1970' as DateTime),Getdate()) AgeQuarters