@Chanwan
Here's the detail of all Slam finalists since the start of 1991 (101 Slams in that time):
Age of Winners
19: 1 (Nadal, RG 2005).
20: 7 (Courier, RG 1991; Kuerten, RG 1997; Safin, UO 2000; Hewitt, UO 2001; Nadal, RG 2006; Djokovic, AO 2008; Del Potro, UO 2009).
21: 8 (Courier, AO 1992; Courier, RG 1992; Sampras, WI 1993; Moya, RG 1998; Hewitt, WI 2002; Federer, WI 2003; Roddick, UO 2003; Nadal, RG 2007).
22: 13 (Stich, WI 1991; Agassi, WI 1992; Courier, AO 1993; Bruguera, RG 1993; Sampras, UO 1993; Sampras AO 1994; Sampras WI 1994; Kafelnikov, RG 1996; Federer, AO 2004; Federer, WI 2004; Nadal, RG 2008; Nadal, WI 2008; Nadal, AO 2009).
23: 8 (Becker, AO 1991; Bruguera, RG 1994; Sampras, WI 1995; Kuerten, RG 2000; Ferrero, RG 2003; Federer, UO 2004; Federer, WI 2005; Djokovic, AO 2011).
24: 16 (Agassi, UO 1994; Agassi, AO 1995; Sampras, UO 1995; Krajicek, WI 1996; Rafter, UO 1997; Kafelnikov, AO 1999; Kuerten, RG 2001; Federer, UO 2005; Federer, AO 2006; Federer, WI 2006; Nadal, RG 2010; Nadal, WI 2010; Nadal, UO 2010; Djokovic, WI 2011; Djokovic, UO 2011; Djokovic, AO 2012).
25: 14 (Edberg, UO 1991; Sampras, UO 1996; Sampras, AO 1997; Sampras, WI 1997; Rafter, UO 1998; Gaudio, RG 2004; Safin, AO 2005; Federer, UO 2006; Federer, AO 2007; Federer, WI 2007; Nadal, RG 2011; Murray, UO 2012; Djokovic, AO 2013; Cilic, UO 2014).
26: 7 (Edberg, UO 1992; Sampras, WI 1998; Johansson, AO 2002; Costa, RG 2002; Federer, UO 2007; Nadal, RG 2012; Murray, WI 2013).
27: 9 (Muster, RG 1995; Sampras, WI 1999; Federer, UO 2008; Federer, RG 2009; Federer, WI 2009; Nadal, RG 2013; Nadal, UO 2013; Djokovic, WI 2014; Djokovic, AO 2015).
28: 8 (Becker, AO 1996; Sampras, WI 2000; Federer, AO 2010; Wawrinka, AO 2014; Nadal, RG 2014; Djokovic, WI 2015; Djokovic, UO 2015; Djokovic, AO 2016).
29: 4 (Agassi, RG 1999; Agassi, UO 1999; Agassi, AO 2000; Ivanisevic, WI 2001).
30: 4 (Korda, AO 1998; Agassi, AO 2001; Federer, WI 2012; Wawrinka, RG 2015).
31: 1 (Sampras, UO 2002).
32: 1 (Agassi, AO 2003).
Age of Runners-Up
20: 7 (Ivanisevic, WI 1992; Berasategui, RG 1994; Moya, AO 1997; Nalbandian, WI 2002; Baghdatis, AO 2006; Nadal, WI 2006; Djokovic, UO 2007).
21: 7 (Agassi, RG 1991; Courier, UO 1991; Sampras, UO 1992; Philippoussis, UO 1998; Roddick, WI 2004; Nadal, WI 2007; Murray, UO 2008).
22: 10 (Courier, RG 1993; Courier, WI 1993; Ivanisevic, WI 1994; Rios, AO 1998; Safin, AO 2002; Ferrero, RG 2002; Coria, RG 2004; Roddick, WI 2005; Tsonga, AO 2008; Murray, AO 2010).
23: 11 (Becker, WI 1991; Martin, AO 1994; Sampras, AO 1995; Chang, RG 1995; Chang, AO 1996; Clement, AO 2001; Ferrero, UO 2003; Hewitt, UO 2004; Hewitt, AO 2005; Djokovic, UO 2010; Murray, AO 2011).
24: 15 (Korda, RG 1992; Pioline, UO 1993; Chang, UO 1996; Rusedski, UO 1997; Corretja, RG 1998; Enqvist, AO 1999; Medvedev, RG 1999; Norman, RG 2000; Verkerk, RG 2003; Safin, AO 2004; Federer, RG 2006; Roddick, UO 2006; Soderling, RG 2009; Berdych, WI 2010; Nishikori, UO 2014).
25: 12 (Stich, UO 1994; Agassi, UO 1995; Kafelnikov, AO 2000; Federer, RG 2007; Soderling, RG 2010; Nadal, WI 2011; Nadal, UO 2011; Nadal, AO 2012; Djokovic, RG 2012; Murray, WI 2012; Djokovic, UO 2012; Murray, AO 2013).
26: 12 (Edberg, AO 1992; Bruguera, RG 1997; Ivanisevic, WI 1998; Schuettler, AO 2003; Philippoussis, WI 2003; Puerta, RG 2005; Gonzalez, AO 2007; Federer, RG 2008; Federer, WI 2008; Roddick, WI 2009; Djokovic, WI 2013; Djokovic, UO 2013).
27: 10 (Edberg, AO 1993; Becker, WI 1995; Stich, RG 1996; Washington, WI 1996; Rafter, WI 2000; Corretja, RG 2001; Federer, AO 2009; Nadal, AO 2014; Djokovic, RG 2014; Murray, AO 2015).
28: 5 (Pioline, WI 1997; Rafter, WI 2001; Federer, UO 2009; Djokovic, RG 2015; Murray, AO 2016).
29: 4 (Agassi, WI 1999; Martin, UO 1999; Sampras, UO 2000; Federer, RG 2011).
30: 2 (Lendl, AO 1991; Sampras, UO 2001).
31: 1 (Ferrer, RG 2013).
32: 2 (Agassi, UO 2002; Federer, WI 2014).
33: 1 (Federer, WI 2015).
34: 1 (Federer, UO 2015).
35: 1 (Agassi, UO 2005).
Age of All Finalists
19: 1
20: 14
21: 15
22: 23
23: 19
24: 31
25: 26
26: 19
27: 19
28: 13
29: 8
30: 6
31: 2
32: 3
33: 1
34: 1
35: 1