Mario ages 29 and 30 is still elite offensively, and especially on the PP but as an ES player he no longer was.
I haven't yet done the tables for years prior to 87-88 (this is a condensed version of what I put together for the Yzerman v. Sakic thread) but here are some ESP/60 leaders so you can get an idea of how well Lemieux compares to other players. I've made sure to include the leaders, relevant players close to Lemieux, and Lemieux's most "career competitors" in Gretzky, Messier, and Yzerman.
Year|Player|Pos|GP|ESP|ESPPG
1987-88|Wayne Gretzky|C|64|91|1.42
1987-88|Mario Lemieux|C|77|74|0.96
1987-88|Steve Yzerman|C|64|61|0.95
1987-88|Mark Messier|C|77|70|0.91
1987-88|Pat LaFontaine|C|75|61|0.81
1987-88|Jari Kurri|R|80|61|0.76
1987-88|Paul Coffey|D|46|21|0.46
Year|Player|Pos|GP|ESP|ESPPG
1988-89|Mario Lemieux|C|76|102|1.34
1988-89|Wayne Gretzky|C|78|100|1.28
1988-89|Steve Yzerman|C|80|101|1.26
1988-89|Bernie Nicholls|C|79|87|1.10
1988-89|Rob Brown|R|68|68|1.00
1988-89|Luc Robitaille|L|78|74|0.95
1988-89|Jari Kurri|R|76|65|0.86
1988-89|Jimmy Carson|C|80|65|0.81
1988-89|Mark Messier|C|72|46|0.64
Year|Player|Pos|GP|ESP|ESPPG
1989-90|Wayne Gretzky|C|73|96|1.32
1989-90|Mario Lemieux|C|59|71|1.20
1989-90|Steve Yzerman|C|79|79|1.00
1989-90|Mark Messier|C|79|71|0.90
1989-90|Brett Hull|R|80|69|0.86
1989-90|Pat LaFontaine|C|74|63|0.85
1989-90|Luc Robitaille|L|80|67|0.84
Year|Player|Pos|GP|ESP|ESPPG
1990-91|Wayne Gretzky|C|78|103|1.32
1990-91|Adam Oates|C|61|73|1.20
1990-91|Mario Lemieux|C|26|31|1.19
1990-91|Brett Hull|R|78|86|1.10
1990-91|Cam Neely|R|69|61|0.88
1990-91|Luc Robitaille|L|76|68|0.89
1990-91|Joe Sakic|C|80|71|0.89
1990-91|Steve Yzerman|C|80|68|0.85
1990-91|Alexander Mogilny|F|62|51|0.82
1990-91|Mark Messier|C|53|42|0.79
Year|Player|Pos|GP|ESP|ESPPG
1991-92|Mario Lemieux|C|64|74|1.16
1991-92|Kevin Stevens|L|80|81|1.01
1991-92|Brett Hull|R|73|64|0.88
1991-92|Adam Oates (STL)|C|54|46|0.85
1991-92|Wayne Gretzky|C|74|63|0.85
1991-92|Steve Yzerman|C|79|66|0.84
1991-92|Mark Messier|C|79|65|0.82
Year|Player|Pos|GP|ESP|ESPPG
1992-93|Mario Lemieux|C|60|96|1.60
1992-93|Steve Yzerman|C|84|87|1.04
1992-93|Alexander Mogilny|R|77|79|1.03
1992-93|Pat LaFontaine|C|84|83|0.99
1992-93|Teemu Selanne|R|84|83|0.99
1992-93|Kevin Stevens|L|72|67|0.93
1992-93|Adam Oates|C|84|78|0.93
1992-93|Luc Robitaille|L|84|76|0.90
1992-93|Jaromir Jagr|R|81|71|0.88
1992-93|Wayne Gretzky|C|45|38|0.84
1992-93|Pavel Bure|R|83|68|0.82
1992-93|Joe Juneau|L|84|65|0.77
1992-93|Mark Messier|C|75|57|0.76
1992-93|Doug Gilmour|C|83|63|0.76
Year|Player|Pos|GP|ESP|ESPPG
1993-94|Eric Lindros|C|65|67|1.03
1993-94|Mario Lemieux|C|22|22|1.00
1993-94|Sergei Fedorov|C|82|81|0.99
1993-94|Cam Neely|R|49|47|0.96
1993-94|Steve Yzerman|C|58|52|0.90
1993-94|Jaromir Jagr|R|80|70|0.88
1993-94|Adam Oates|C|77|62|0.81
1993-94|Wayne Gretzky|C|81|62|0.77
1993-94|Pavel Bure|R|76|58|0.76
1993-94|Doug Gilmour|C|83|60|0.72
1993-94|Mark Messier|C|76|39|0.51
Year|Player|Pos|GP|ESP|ESPPG
1995-96|Jaromir Jagr|R|82|95|1.16
1995-96|Mario Lemieux|F|70|73|1.04
1995-96|Eric Lindros|C|73|75|1.03
1995-96|Alexander Mogilny|R|79|71|0.90
1995-96|Peter Forsberg|C|82|70|0.85
1995-96|Sergei Fedorov|C|78|65|0.83
1995-96|Ron Francis|C|77|63|0.82
1995-96|Mark Messier|C|74|58|0.78
1995-96|Wayne Gretzky|C|80|54|0.68
1995-96|Steve Yzerman|C|80|50|0.63
Year|Player|Pos|GP|ESP|ESPPG
1996-97|Eric Lindros|C|52|56|1.10
1996-97|Jaromir Jagr|R|63|67|1.06
1996-97|Mario Lemieux|C|76|79|1.04
1996-97|John Leclair|L|82|81|0.99
1996-97|Teemu Selanne|R|78|76|0.97
1996-97|Paul Kariya|L|69|61|0.88
1996-97|Peter Forsberg|C|65|53|0.82
1996-97|Wayne Gretzky|C|82|66|0.80
1996-97|Mark Messier|C|71|55|0.77
1996-97|Steve Yzerman|C|81|57|0.70
Year|Player|Pos|GP|ESP|ESPPG
2000-01|Mario Lemieux|C|43|43|1.00
2000-01|Jaromir Jagr|R|81|78|0.96
2000-01|Joe Sakic|C|82|66|0.80
2000-01|Steve Yzerman|C|54|31|0.57
2000-01|Mark Messier|C|82|32|0.39
Year|Player|Pos|GP|ESP|ESPPG
2001-02|Alexei Kovalev|F|67|53|0.79
2001-02|Jaromir Jagr|R|69|54|0.78
2001-02|Jarome Iginla|R|82|64|0.78
2001-02|Todd Bertuzzi|R|72|55|0.76
2001-02|Markus Naslund|L|81|59|0.73
2001-02|Mario Lemieux|C|24|17|0.71
2001-02|Steve Yzerman|C|52|24|0.46
2001-02|Mark Messier|C|41|14|0.34
Year|Player|Pos|GP|ESP|ESPPG
2002-03|Peter Forsberg|F|75|73|0.97
2002-03|Joe Thornton|C|77|65|0.84
2002-03|Milan Hejduk|R|82|63|0.77
2002-03|Glen Murray|R|82|63|0.77
2002-03|Mario Lemieux|C|67|46|0.69
2002-03|Joe Sakic|C|58|39|0.67
2002-03|Markus Naslund|L|82|50|0.61
2002-03|Steve Yzerman|F|16|5|0.31
2002-03|Mark Messier|C|78|23|0.29
Year|Player|Pos|GP|ESP|ESPPG
2003-04|Peter Forsberg|C|39|30|0.77
2003-04|Alex Tanguay|L|69|53|0.77
2003-04|Markus Naslund|L|78|58|0.74
2003-04|Joe Thornton|C|77|57|0.74
2003-04|Patrik Elias|L|82|57|0.70
2003-04|Mario Lemieux|C|5|10|0.50
2003-04|Mark Messier|C|76|35|0.46
2003-04|Steve Yzerman|F|75|33|0.44
Year|Player|Pos|GP|ESP|ESPPG
2005-06|Joe Thornton|C|81|72|0.89
2005-06|Jaromir Jagr|R|82|71|0.87
2005-06|Steve Yzerman|F|61|23|0.38
2005-06|Mario Lemieux|F|22|7|0.32
As you can see, Lemieux doesn't drop from the league leaders in ES offense until his mid-30s. Given that he often scored half or more than half of his points on the PP (Lemieux is arguably the greatest PP player ever), that's pretty significant with regards to overall domination; ES is the most important situation as it's the most common, but a guy who is insanely effective on the PP as well as being one of the league's top three ES scorers is extremely valuable.