When does a car start to show its age?

It’s really difficult to notice, I think you’d have to do a back to back with a much newer and/or lower mileage version. I suspect the ride has deteriorated in the BMW over time(it’s on 96k miles) as it’s not quite as smooth as I think it used to be, but that could just be due to driving other cars recently. Everything else feels as it always has and better than it did over the winter.

It's difficult to detect. My Legacy needed a suspension refresh before I got shot of it as it was noticeably low at the rear and I suspect a lot of the standard bushes had had it. I didn't realise that the TVR suspension had seen better days until I put the new stuff on.

Yes, mine will be due some new Bilsteins on the back at some point fairly soon, to complement the new fronts last summer.

A significant factor in what makes a car feel it's age is something that is difficult to get some quality data on.

As a car ages, it's structure is subject to significant load during day to day use that weaken the structure, by which I mean the metal and the spot welds (extremes of temperature and climate, daily suspension loads and the odd massive pot hole, vibrations and loads from the powertrain, etc).

I do know that early carbon fibre tubs (such as that used on the update EB110) would lose 30% of their structural strength just through ageing alone (10 year period). I am not a materials engineer, so I don't know how steel or aluminium compares - but I would expect that the body shell of a 5 year old car is noticeably less rigid than when it was factory fresh.

If this is the case - you can change all the suspension bits and pieces you want, the car will never drive or feel like it did when it was new.

Apart from some tiredness in the suspension and some rust, my Honda felt almost as tight at 18 years of age as it did when new.
The 820 felt tired in suspension and engine department when I traded it in at 8 years of age, but I don't recall any rust. Both the above cars had done over 100,000 when I changed them

I think it varies a lot with the care and use of it, as well as the type. But I do think somewhere between 60-80k miles, and for me around 6 years old, you do start to feel changes usually.

The XFR looks mint outside and in, besides some stone chipping (nothing like as bad on some cars I've had, but it's always there on the nose and screen of high mileage cars) but mechanically the steering and suspension feel their age. I noticed it really badly after the Jag test day I did. The 330i was the same whenever I had a nearly new 3 series loan car. There is just some bagginess in the steering and suspension movement, some vibrations on some surfaces that feel a bit like a slightly warped disc. Occasional but there if you look for it! The Passat V6, A4 2.0T, and 156GTA were much the same, but whereas the drivetrains of the others all felt fine, the A4 started some turbo weirdness around 70k miles that changing some of the bypass seals and so on did not totally cure - it hung on to revs in gearchanges in a weird way that only familiarity with it would have revealed.

Of that lot, the 330i and the Passat's Audi derived V6 ran fine but both started using oil, both burning it and dripping it slightly, well before 6 figure mileages. Changing stuff (the BMW was still under warranty) did not fix it and BMW straight 6 petrols are known for their "burning candle" smell at higher miles. Small amounts, but there. The XFR V8 is not using oil at all and of all the engines I've used feels the tightest at higher mileages. Impressive.

Of Mrs ALF's cars, the current C Max was rattling a lot inside from less than 3 years/30k (which, curable driver's window on the XFR aside, my cars have not done at far higher mileages). It also has some odd turbo induction/wastegate stuff going on.The Focus was a totall mess inside, falling apart, but the 2 litre NA petrol engine and the gearbox felt spot on even 8 years old, and never used oil - neither did the Civic Type R or Gold 1.6FSi - although the latter had an appetite for EDC valves.

For me, they can look great, but by 30m ish miles and a couple of years old you can spot differences with a new car. As you get towards 6 figure mileages and certainly by 6-8 years old there is bagginess there, but in isolation the car will feel fine if looked after. It's just very different from a new one. That's one reason I tend to buy around 2-4 years old and keep for 3 ish years. The other being depreciation.