At what point is it fair to call a device "too old" to support? 3 years? 4? 5?
It's so impressive what you can do in experiences with modern hardware and graphical capabilities. But it's all a balance of give and take. How many potential users are you alienating by upping the minimum hardware requirements?
It's a broad question, but focusing on Roblox (something I'm very familiar with), actually has a lot more to do with the type of audience you're trying to attract.
The average American consumer will keep a smartphone for almost 3 years (2.75 years to be exact). However, using this figure to base your "too old" guess on would be misleading. Most Roblox players are younger - and young kids aren't the ones running out to buy the new iPhone. Instead, that's what their parents do - and that means the parents' old phone is what's being handed down. So that 3-year buffer of avg ownership looks more like 6 years when you consider how long parents will hold their new device for. A 6-year pad would put the typical device generation that many kids are playing Roblox on as roughly 2015. The iPhone 6S/7 generation.
However, a lot of kids will also play on iPads or tablets. Furthermore, unlike phones, kids will typically receive their own iPad as a gift just for their use. So the hand-me-down approach of phones doesn't traditionally apply. However, average iPad ownership ALSO roughly syncs up at 6 years - conveniently. That means our target for the phone-based demographic lines up almost perfectly with our tablet-based one.
And all of this lines up with the devices we typically have seen in our experiences.
While this might seem like an EXTREMELY convoluted way of figuring this out - it has merit.
When you're designing an experience and want to target a generation newer than the average device, you know that your audience will likely age up from the norm. So you'll be targeting young or mid-teens, and they have very different preferences from the ones on their parent's old 6S Plus.
So the answer? It entirely depends on what your experience is trying to achieve. For example, suppose you're designing an experience where you're selling virtual goods to an audience that may not necessarily be Roblox-native. In that case, you're probably safe to only target the iPhone 8-era. The up-leveled graphics capability may help you get some added media attention - even.
However, suppose you're designing the 50th pet collection tycoon simulator world with a limited color pallet and a square sun. In that case, you'll probably want to hold on to those old devices for a bit longer.
Aim for the demographic you're trying to it.