It's nonsense that only newer PBs win. The only reason that more newer ones win is because there is more of them.
Plenty of people have £50k's worth now, but in decades gone by, that was an inconceivably vast sum to the majority. Set for life money. Now it's a house deposit or cost of a modest extension. But if you look at the list of prizes, there's often someone who's had a few quid for decades who's won a big prize.
Cashing them in and buying new ones reduces your chance of winning because you're out of the draw for a month.
There's a lot of myths about PBs because most people don't understand them. Moneysaving Expert engaged a mathematician to calculate the statistics and there's some very interesting numbers.
For example, if you hold the maximum £50k for a year, statistically, you have a 99.4% chance of winning at least £750 and on average will win £1600, so a payout of 3.2%, which is beaten by the best instant access savings account, but not a bad option if you're a higher rate taxpayer who's already used up their tax free savings allowances.
For the person who says her DM had the maximum amount for 20 years without winning anything, that is statistically impossible, you must either be mistaken, your DM must be lying, or she will have not updated her details - there are millions of unclaimed prizes.