记住，他才14岁。用英语怎么说？

记住，他才14岁。

Remember he's just fourteen years old.

remember

vt.& vi. 记得，牢记；vt. 记着，纪念，回想起，送钱[礼物]给…；

例句
Although I was only six, I can remember seeing it on TV...
虽然那时我只有6岁，我依然记得在电视上见过它。

just

adv. 刚才，仅仅，只是，正好，刚要；adj. 公正的，合理的，恰当的，合法的，正确的；

例句
It became very clear that the incident was not just an aberration, it was not just a single incident.
事实已经很清楚，这件事不是偶发的，它并不是一起孤立的事件。

fourteen

num. 十四，十四个，第十四；n. 十四的记号，十四岁，十四点钟，十五世纪；

例句
Oil prices fell this week to their lowest level in fourteen months, apparently because of over-production.
据说因为产量过剩，这个星期石油价格跌至14个月以来的最低点。

years

n. 年( year的名词复数 )，年纪，一年的期间，某年级的学生；

例句
His wife was sentenced to seven years imprisonment for aiding and abetting him...
他的妻子作为他的从犯被判入狱7年。

old

adj. 老的，古老的，以前的，（用于指称被替代的东西）原来的；n. 古时；

例句
She was old enough to absent herself from the lunch table if she chose...
她长大了，可以自己做主不来吃午餐了。