Are they reaching for the salt or pepper shaker and knocking it over all the time? Have they started eating a lot more than usual? Are they having "mood swings" more often than when they were younger? Are they having privacy issues?
Puberty happens in different children at different times - the usual time is somewhere in that six-month period before their 13th birthday. Usually their sweat glands begin to produce perspiration for the first time in their lives so their "smell" will change - and quite abruptly. Some children are "late-bloomers" and won't enter puberty until after they turn 13 - a lot of it seems to be influenced by diet nowadays. It will be a struggle for all of you in the household - just hang on and the child will settle down.