30 years old isn't old yet.
the folks (kids) asking about the difference between 20 y/o and 30 y/o are closer to the 20 than 30 I would imagine. 30 isn't old.
Here are the fastest 5k run times for each age. You can see there's very very little drop off from age 20 to age 30 and very little slow down through the upper 30s.
20y089d 12:45.09 Daniel Komen (KEN) 17 May 1976 14 Aug 1996 Zurich SUI
21y353d 12:37.35 Kenenisa Bekele Beyeche (ETH) 13 Jun 1982 31 May 2004 Hengelo NED
22y120d 12:44.39 Haile Gebreselasie (ETH) 18 Apr 1973 16 Aug 1995 Zurich SUI
23y018d 12:40.18 Kenenisa Bekele Beyeche (ETH) 13 Jun 1982 01 Jul 2005 Saint Denis FRA
24y117d 12:41.86 Haile Gebreselasie (ETH) 18 Apr 1973 13 Aug 1997 Zurich SUI
25y056d 12:39.36 Haile Gebreselasie (ETH) 18 Apr 1973 13 Jun 1998 Helsinki FIN
26y115d 12:49.64 Haile Gebreselasie (ETH) 18 Apr 1973 11 Aug 1999 Zurich SUI
27y138d 12:51.34 Edwin Cheruiyot Soi (KEN) 03 Mar 1986 19 Jul 2013 Fontvieille MON
28y057d 12:49.87 Paul Tergat (KEN) 17 Jun 1969 13 Aug 1997 Zurich SUI
29y320d 12:49.71 Mohamed Mourhit (BEL) 10 Oct 1970 25 Aug 2000 Brussels BEL
30y174d 12:49.04 Thomas Pkemoi Longosiwa (KEN) 14 Jan 1982 06 Jul 2012 Saint Denis FRA
31y013d 12:55.18 Paul Tergat (KEN) 17 Jun 1969 30 Jun 2000 Rome ITA
32y132d 12:49.28 Brahim Lahlafi (MAR) 15 Apr 1968 25 Aug 2000 Brussels BEL
33y019d 12:57.62 Mark Kiptoo Kosgei (KEN) 21 Jun 1976 10 Jul 2009 Rome ITA
34y046d 12:53.46 Mark Kiptoo Kosgei (KEN) 21 Jun 1976 06 Aug 2010 Stockholm SWE
35y174d 12:54.12 Bernard Lagat (USA) 12 Dec 1974 04 Jun 2010 Oslo NOR
36y222d 12:53.60 Bernard Lagat (USA) 12 Dec 1974 22 Jul 2011 Fontvieille MON
37y262d 12:59.92 Bernard Lagat (USA) 12 Dec 1974 30 Aug 2012 Zurich SUI
38y268d 12:58.99 Bernard Lagat (USA) 12 Dec 1974 06 Sep 2013 Brussels BEL
39y262d 13:06.68 Bernard Lagat (USA) 12 Dec 1974 31 Aug 2014 Berlin GER
40y168d 13:14.97 Bernard Lagat (USA) 12 Dec 1974 29 May 2015 Eugene OR USA
41y252d 13:06.78 Bernard Lagat (USA) 12 Dec 1974 20 Aug 2016 Rio de Janeiro B
The difference is the 30 year old didn't get up off the couch after 10 years of inactivity and run a sub 13 5k. He was performing at an elite level all his life. A 20 year old can get off the couch and put the cheetos away, and while he's waited too late to become a world class runner, he can improve faster than a sedentary 30 y/o would improve.
But sherdog has some weird ideas about who is old and who isn't.