The rating of a television show can refer to two different things--its Nielson rating, meaning how popular it is, and its content rating, which refers to how mature the show is. Unfortunately, both of these ratings change from one episode to another, so there is no blanket statement for either rating regarding The Mentalist as a whole.
The Mentalist - 2008 The Thin Red Line 1-8 is rated/received certificates of:
Netherlands:12
Line - 1961 is rated/received certificates of: Finland:K-16 Norway:15
Murder on Line One - 1989 is rated/received certificates of: Germany:BPjM Restricted
The Unusuals - 2009 The Apology Line 1-9 is rated/received certificates of: Netherlands:12
Drive - 2007 The Starting Line 1-1 is rated/received certificates of: USA:TV-14
Necessary Roughness - 2011 Goal Line 1-12 is rated/received certificates of: Netherlands:6
On the Line - 2008 I is rated/received certificates of: Spain:13
Blue Line - 2008 is rated/received certificates of: UK:U
End of the Line - 2008 SUSPENDED is rated/received certificates of: USA:R
Crossing the Line - 2008 III is rated/received certificates of: Australia:PG (self applied)
Flashpoint - 2008 First in Line 1-2 is rated/received certificates of: Netherlands:12 USA:TV-PG
The Suite Life on Deck - 2008 In the Line of Duty 2-3 is rated/received certificates of: USA:G
The Other End of the Line - 2008 is rated/received certificates of: Argentina:13 Australia:PG Mexico:B Netherlands:AL Portugal:M/6 Singapore:PG South Korea:15 USA:PG-13
Kamen Rider Dragon Knight - 2008 Letter from the Front Line 1-20 is rated/received certificates of: USA:TV-Y7
Line by Line - 2010 is rated/received certificates of: Hong Kong:IIA
On the Line - 1971 is rated/received certificates of: USA:G
On the Line - 2011 I is rated/received certificates of: USA:TV-G
On the Line - 2011 II is rated/received certificates of: Canada:G