What and when is Oldhammer?
I guess it is different for each person.
For me…
Well in terms of model and rules age.
WFRP 1st edition.
Rogue Trader for 40k.
For WFB it is a bit different, for rules it is WFB 3rd edition but I feel that for some armies the models from 4th edition better match some of the illustrations from Warhammer Armies, so I would in some cases class WFB 4th edition era models as oldhammer, but not 4th edition rules.
WFB 4th saw the era of super capitalism arrive.
Boxed games, starter sets, an army book for each army, models costing more to buy based on stats regardless of cost to produce, named models with their own background and special rules.
For me 4th edition very much ushered in the era of here is our completed world with lore, now play as we say as it is law.
3rd edition felt very much more here is a rough setting with a tool box of rules, go have some fun doing what you will with it.
With 3rd you could field any character from the setting with a bit of creative thinking, no need to await 4th edition and the mad over powered special rules for named characters.
With 3rd the world was still unexplored and GW invited you to fill in the blanks and go create your own take on the setting.
Warhammer armies also made it clear to house rule stuff as you saw fit, it talked about limiting magic, magical items etc if you wanted more infantry focused games.
TLDR:
For rules that allow you to do what you want, WFRP 1st, WFB 3rd, and Rogue Trader.
For models, the RT adjacent models, and up to some WFB 4th edition era models.
YMMV