BakoSaint

Alvin Kamara is 29 years old. Lets take a factual look at some other great historic running backs and saints running backs and how they faired in rushing yardage (as a simply proxy) from age 25-29 and then age 30-34 to get an idea of projected decline:

Alvin Kamara:
25-29: 932, 898, 897, 694, TBD
30-34: TBD

Marshall Faulk:
25-29: 1319, 1381, 1359, 1382, 953
30-34: 818, 774, 292, retired.

Ladanian Tomlinson:
25-29: 1335, 1462, 1815, 1474, 1110
30-34: 730, 914, 280, retired.

Adrian Peterson:
25-29: 1298, 970, 2097, 1266, 75(inj)
30-34: 1485, 72(inj), 529, 1042, 898

Edgerrin James:
25-29: 1259, 1548, 1506, 1159, 1222
30-34: 514, 125, retired

Shaun Alexander:
25-29: 1175, 1435, 1696, 1880, 896
30-34: 716, 24, retired

Jamal Charles:
25-29: 83(inj), 1509, 1287, 1033, 364
30-34: 40, 296, 7, retired

Lesean McCoy:
25-29: 1607, 1319, 895, 1267, 1138
30-34: 514, 465, 31, retired

Leveon Bell:
25-29: 1291, held out, 789, 328, 101
30-34: retired

Ricky Williams:
25-29: 1853, 1372, took year off, 743, suspended
30-34: 15, 659, 1121, 673, 444

Deuce McAllister:
25-29: 1654, 1074, 335(inj), 1057, 92
30-34: 418, retired

Reggie Bush:
25-29: 150(inj), 1086, 986, 1006, 297
30-34: 28, -3, retired

Mark Ingram II:
25-29: 964, 769, 1043, 1124, 645
30-34: 1018, 299, 554, 233, retired

Marshawn Lynch:
25-29: 1204, 1590, 1257, 1306, 417
30-34: took year off, 891, 376, 34, retired

Thurman Thomas:
25-29: 1407, 1487, 1315, 1093, 1005
30-34: 1033, 643, 381, 152, 136

Barry Sanders:
25-29: 1115, 1883, 1500, 1553, 2053
30-34: 1491 with lowest yards per carry in 6 years by large margin, timed it perfect based on stats and retired.

There are some rare exceptions:

Curtis Martin:
25-29: 1287, 1464, 1204, 1513, 1094
30-34: 1308, 1697, 735 (w/ 3.3ypc), retired

Frank Gore:
25-29: 1036, 1120, 853, 1211, 1214
30-34: 1128, 1106, 967, 1025, 961

Emmitt Smith:
25-29: 1484, 1773, 1204, 1074, 1332
30-34: 1397, 1203, 1021, 975, 256

The most receiving yards any of these three exceptions had in a year after turning 30 was 277. The exceptions seem to be more of the bell cow RB type who are able to sustain above averageness by knowing the system and vision and such, not runners who rely on separation and burst.

I think realistically both the Saints and 49ers would be smart to realize that as brilliant as CMC and Kamara have been in the past, maybe having talent comperable to LaDanian Tomlinson, Marshall Faulk, and Edgerrin James brilliant careers, all those players declined a great deal after turning 30, and it is highly likely Kamara and CMC will too. It will not be a shock if Kamara has a good year at 29, but the odds have to be about 75% that he will decline greatly if they try to give him big money for year 31.

Pay da man means ignore the stats and history.