Rashford still has at minimum 13 PL games, 2 CL games and 1 FA cup (16 total) games to play this season where he turned age 21. With that note in mind here are some charts.
The top 5 youngest to 100 + Shearer
As you can see, Rashford has started slower than some of peers when age adjusted, though he still sits 1 goal ahead of Kane at the same age and only needs 7 more to match Lukaku. Rashford still has 13 games in hand for this season to add to that tally. Rashford has also been played away from CF unlike his peers Its also evident that the teams his peers were playing in were scoring more goals than United have been for the last 4 years.
Also. Shearer was beast mode.
Rooney v Rashford for United, by age.
When comparing these two, you can see that Rashford is currently 18 goals behind Rooney at this age. He does still have at minimum 16 games to play. If we're generous and say he nets 1 in 2, he ends the season 10 goals behind Rooneys total. By no means an insurmountable feat over the next decade given that Rooney only ever hit 20+ goals in all competitions 4 times in a United career spanning 13 years, a fact I still find incredible for the clubs leading ever goal scorer.
If Rashford can stay fit and we continue to play actual football and score goals, he can definitely do it. It's a long road though. Two things in his favour are that his career should go on longer than Rooneys, and he's possibly a one team man.
Rooneys best seasons were the two in which he turned 24 and 26 in which he scored 34 goals. After those two, he only managed 16,19,14,15,& 8 goals in all comps for us in a final season aged 31.