Short answer: about a decade, give or take several years. New Jersey's aged fish of 40 to 43 inches were most often 10, the average Massachusetts striper does not reach 40 inches until about 14, and on Massachusetts' chart 40-inch fish turn up at every age from 8 to 18. That spread is the real answer. A striper adds length fast when young and slower with every inch past the mid-30s, so the tape stops working as a clock right where the fish get interesting.
Age at length, measured
The most direct answer comes from New Jersey. The state's Division of Fish & Wildlife aged 2,170 striped bass by their scales, from spring tagging in Delaware Bay, its bonus-fish program and a fall party-boat survey, and in 2002 its biologist Tom Baum published the ages found in each length group:
| Length | Ages found | Most common age |
|---|---|---|
| 28–31 in | 5–10 | 7 (36%) |
| 32–35 in | 5–11 | 7 and 8 (33% each) |
| 36–39 in | 7–12 | 9 (31%) |
| 40–43 in | 8–13 | 10 (32%) |
| 44–48 in | 9–15 | 11 (31%) |
Read the 40–43 row across and it is wide: 5 percent were 8 years old, 21 percent 9, 32 percent 10, 21 percent 11, 15 percent 12 and 5 percent 13. No single age held even a third of the fish.
The top row sits close to the Massachusetts slot, 28 to under 31 inches. Two thirds of those fish were 6 or 7.
Massachusetts' own chart
The Massachusetts Division of Marine Fisheries publishes a chart called "How old is my striper?", built on nearly 8,000 fish aged in its lab. Every fish is a dot, and an "average" line runs through them. Read off that line, the average Massachusetts striper reaches:
| Length | At about age |
|---|---|
| 28 in | 6 |
| 31 in | 7½ |
| 35 in | 10 |
| 40 in | 14 |
Those are our readings of a published picture, not the agency's own table. And at 40 inches, the dots run across every age from 8 to 18.
So which is it, 10 or 14? Both, because they answer different questions. The line is the average length of fish of one age; the New Jersey table is the ages of fish of one length. Younger year classes usually hold more fish, since some die every year, so the fast growers among the 10-year-olds can outnumber the 14-year-olds that reached 40 inches on schedule. That is arithmetic, not biology, and the samples come from different waters and years. Where they agree is what matters: one length, many ages.
How fast they grow
The same line answers how fast striped bass grow. Going from 28 to 35 inches takes three or four years. The next five inches take about four more. Past 40, the average line nearly flattens.
Slow is not stopped. In otolith-aged fish from Massachusetts to Virginia, Schiano and colleagues found females still gaining length and weight at 22 or older.
Has growth slowed?
For young fish, yes. In Maryland's spawning survey, 36,648 fish from 1985 to 2022, Nelson, Smith and Quinn found length at age falling for fish 7 and younger, with females aged 6 to 8 shortest in the most recent decade.
For big fish, the newest work points the other way. Schiano's team, with data from 1998 to 2019, found females aged 10 to 15 averaged 7.8 cm, about 3 inches, longer at age by the end. The studies differ in years and aging method, and Nelson's team discusses the gap. "Growth has slowed" holds for young Chesapeake fish; it is not established for the ones you would call big.
How many pounds is a 40-inch striper?
New Jersey weighed 1,542 fish. Its predicted weight for a 40-inch striper was 24 pounds, and the 40-inch fish it actually weighed ran from 18 to 37. For a 30-inch fish the prediction was 12 pounds, with a range of 8 to 18.
Schiano's team also found that fish of a given length weighed more at the end of their record than at the start, so a chart from around 2000 may run light today. That last step is our inference.
The oldest fish read young
New Jersey's ages, like Maryland's, come from scales. At 40 inches that is probably fine. When Secor and colleagues aged Maryland trophy-season fish both ways in 1992, scale ages matched otolith (ear-stone) ages on average for fish aged 5 to 11, which measured 36 to 43 inches (91–110 cm). For fish over 47 inches (120 cm), aged 22 to 31 by their otoliths, the scales read 9 years young on average.
In Chesapeake fish of known age, Liao and colleagues found scale ages agreed with the true age 22 percent of the time, against 74 percent for otoliths: scales read young fish old and old fish young. So part of the spread in a scale-based table may be reading error rather than growth.
That covers the 50-inch question too. Fifty inches is off the top of the Massachusetts chart, whose oldest dot sits at age 30. In Secor's sample the fish over 47 inches were 22 to 31, though it held no 12- to 19-year-olds because those year classes were scarce. DMF says striped bass can live more than 30 years.
What to do with it
Treat a length as a range. A 40-inch fish caught this year was probably born somewhere between 2008 and 2018.
Think in year classes. The big 2015 year class averaged about 31½ inches as 8-year-olds in 2023, according to DMF, and turned 11 this year. On the Massachusetts average line, 11-year-olds run about 37 inches, with the fast growers past 40. That is our reading of the chart, not a survey of the class.
Know what the slot's top end is for. A keeper is mostly a 6- or 7-year-old. DMF notes that anglers backed the 31-inch maximum as a way to protect "the largest and most fecund female fish". A 40-incher has lived several more seasons to get there; if you let it go, release it well.
How StripedLines uses this
It doesn't, by design. StripedLines is built for striped bass, and it rates water the same way for bass of every size. When you log a catch you can record a size bucket, up to 40 inches and over; it is saved with your trip. The app can point you at water. It cannot tell you whether the fish there will be 28 inches or 40, or how old the one you land is.