Smaug's age is one possible answer. In Chapter 12 of the Hobbit (Inside Information), he says

The King under the Mountain is dead and where are his kin that dare seek revenge? Girion Lord of Dale is dead, and I have eaten his people like a wolf among sheep, and where are his sons' sons that dare approach me? I kill where I wish and none dare resist. I laid low the warriors of old and their like is not in the world today. Then I was but young and tender.

(Emphasis added.) My reading of this is the Smaug was relatively young when he attacked Erebor. Coming of age, he proved his prowess and gained a large amount of treasure by attacking the greatest (and richest) remaining Dwarf kingdom. Had he attacked years before, he would have been too vulnerable to arrows, or too small to incinerate his enemies in sufficient numbers.

The alternative is that 'warriors of old' whose like 'is not in the world today' refers to individuals alive much earlier, but that seems unlikely because there is no mention of Smaug in the Tale of Years before his attack on Erebor in TA2770. Moreover, had he slain some mighty ancient warriors, he would have boasted about killing them, rather than Girion of Dale.

Smaug's age is one possible answer. In Chapter 12 of the Hobbit (Inside Information), he says

The King under the Mountain is dead and where are his kin that dare seek revenge? Girion Lord of Dale is dead, and I have eaten his people like a wolf among sheep, and where are his sons' sons that dare approach me? I kill where I wish and none dare resist. I laid low the warriors of old and their like is not in the world today. Then I was but young and tender.

(Emphasis added.) My reading of this is the Smaug was relatively young when he attacked Erebor. Coming of age, he proved his prowess and gained a large amount of treasure by attacking the greatest (and richest) remaining Dwarf kingdom. Had he attacked years before, he would have been too vulnerable to arrows, or too small to incinerate his enemies in sufficient numbers.

The alternative is that 'warriors of old' whose like 'is not in the world today' refers to individuals alive much earlier, but that seems unlikely because there is no mention of Smaug in the Tale of Years before his attack on Erebor in TA2770. Moreover, had he slain some mighty ancient warriors, he would name them when boasting, rather than talking about Girion of Dale.