IMHO "20 years" or other lengths of time alone would not be suitable for the retro standard.
Example: the best consumer-grade CPU one could buy when Win3.1 hit the market (April 1992) was 80486DX2-66 (which was not exactly "consumer" back then due to being very expensive and more suitable for workstations rather than mainstream desktop PC; similar to i9, R9, or Threadripper today), but it would be struggling with Win9x and became practically useless with WinXP, which was just 9.5 years (October 2001) from Win3.1. OTOH I just upgraded the Q8300 (first released November 2008) computer of my parents' living room to Win10 and GTS450 earlier this week, making it not only capable of basic office tasks, video streaming, casual games, but could also run older triple-A games like Call of Duty: Black Ops.
Hardware-wise, I'd say there had been three major milestones:
- Integration of EIDE / ATA-2 disk controller (Intel 430FX "Triton"), circa 1994: Socket 5 Pentium, introduction of EDO RAM and PCI local bus, with "Chicago" in the horizon.
- Capable of booting from USB a.k.a. "Legacy-free" by PC System Design Guide 2001 edition: Pentium 4 / AthlonXP, DDR, AGP, WinXP.
- UEFI, defined in 2005 but became mainstream in consumer market only after 2011: Sandy Bridge, DDR3, Win7.
I call anything older than 1 "collectable" i.e. they can be prized collections but with limited practical uses. Working MB, FPM RAM, AT power supply / chassis, and VLB / ISA video cards are hard to find and expensive, plus you've got limits like 528MB / 504MiB.
Systems between 1-2 are "vintage" and are suitable for all but the earliest DOS games and many early Windows (DirectX) games. Limitations exist but can be adapted, converted, or bypassed with readily available kits i.e. no soldering required.
Systems between 2-3 are "retro" and can deal with most late Win9x and WinXP games and apps; later, higher-end models can still deal with modern OS and games except latest ones, such as the Q8300 I mentioned above. Components are either compatible with modern computers, or easy to find and cheap to buy.
Any system with UEFI would be considered "modern" to me, although "modern" does not equal to "suitable for modern gaming." Earlier this year I helped a friend upgrading, recovering and reinstalling an SFF Asus K20DA with A4-6210; his daughter uses it to play Genshin Impact, which is VERY demanding for this APU.