Is it true that young investors, 20s/30s should welcome a recession, since they're looking to grow portfolio and not reap its benefits immediately?

Factors I would consider

1. Investable amount: at low 20s or even 30s, the amount you have to invest might be relatively low, so yes while true that investing at recession/near lows will give outsized percentage returns, important to also think about it absolute dollar wise

2. at this age might be more important to increase income (or even stay in a job) and improve future investable dollars so recession might not be good. While it can also be said that as you reach your prime (40s?) then you are right in the bull market if this recession blows by - hence it will be good

in all really, it’s just hard to time the market and financial planning should be resilient to recession. Having the conviction to stick to a plan matters more

Yes. However, I would argue that age does not matter as much as what most advisors suggest. Many underestimate their holding period. A good financial plan will ensure that age is not relevant in the strategy you employ. It changes as you age. But no matter the age, recessions are welcomed due to opportunities.