I always assumed that players in this game would randomly retire after the age of 30 but after a bit of looking around there is zero info on retirement. It seems that players simply have diminishing physicals from 31 onwards and we retire them whenever we like, correct?

Retirement is random, but this is how training is affected by age ...
The amount of training points players receive each training cycle steadily decreases after the age of 27 and after the players reach 31 they don't receive any training points at all. This is because they need all the training just to sustain their skills. After the age of 31 the players also start to lose points in their physical skills. See the list below for more info.
- Players aged 16 to 27 train normal and lose no points due to their age.
- Players that are 28 years old lose 25% of their training.
- Players that are 29 years old lose 50% of their training.
- Players that are 30 years old lose 75% of their training.
- Players that are 31 years old get no training points. They also lose from 10 to 15 points on Speed, Strength, Agility and Stamina each training cycle.
- Players that are 32 years old get no training points. They also lose from 16 to 20 points on Speed, Strength, Agility and Stamina each training cycle.
- Players that are 33 years old and above get no training points. They also lose from 21 to 25 points on Speed, Strength, Agility and Stamina each training cycle.
NOTE: No player skill can go below 12 because of his age.

hosh13 wrote: So there's nothing in the manual about retirement?

nope - and nothing in the manual about half the game either.

37 I think is the oldest recorded on a human controlled team. There is a 35yo floating around the transfer market recently. Maybe he'll make this 36th year?

But we dont see much go past 31/32. Most managers will drop those players at the end of the 30th year due to lost physicals from 31 onwards.

Most 31yo or older generally are roster cut style retirements.

I think QBs and Ks probably dodge that style of retirement. Although most kickers dont make it past 25 as no one wants to pay their increasing wages.

A season or 2 back I had a Danish NT OLinemen that I kept all season in this 31st year. Was like 17 in spd and 20 in str from memory.

In his 30th year, I trained his physicals so they were like up to 17.90/20.90. As you lose 10-15 (avg 12.5) per week. Took 8 weeks to lower to 19 str. Still acceptable in my books. 16 weeks to lower to 18 str. At which I switched him to guard (backup outside tackle) with the up and coming new OT starter.

He was 17 blocking + other skills. The new kid started the season with 15 skills. But the time the switch occurred, he was running the same skills to be ready for OT.

Sadly the original bloke was sold on at the end of the season.